Polarized Light Digital Image Gallery
Rocks and Minerals
Minerals are naturally occurring substances of inorganic origin that have a definite chemical composition and usually exhibit a crystalline structure. Plants and animals require a wide variety of minerals in order to survive on a daily basis. Minerals are also important, however, as the basic building blocks of rocks, which comprise the solid portion of the Earth and other planets. Rocks are formed through various means and are usually broadly categorized based on this fundamental characteristic. More precisely, igneous rocks are those that have solidified from magma, sedimentary rocks are those that have formed from fragments, or sediments, of other rocks, and metamorphic rocks are produced via the alteration of igneous or sedimentary rocks.
Actinolite Schist - Actinolite is an amphibole mineral abundant in the metamorphic rocks called schists that are easily divided into layers or flakes. The substance, which is typically formed during low-grade metamorphism of certain igneous or carbon-rich rocks, occurs in several different varieties. One type of actinolite, for instance, is known as nephrite, one of the two minerals commonly referred to as jade. Creamy white to green in color, nephrite is more common and less valuable than jadeite, the other form of jade, but, nevertheless, has a long history of ornamental and religious use in various countries, especially China.
Alkalic Granite - Though primarily composed of quartz and feldspars, granite also frequently contains small amounts of hornblende, micas, and other minerals. Moreover, the proportion of plagioclase to alkali feldspar may differ in granite samples, which is often utilized as the starting point for granite classifications. Alkalic, or alkali, granites, for instance, are those in which the alkali feldspar content significantly exceeds the plagioclase. These types of rocks characteristically contain pyroxenes and sodic-amphiboles.
Alkalic Syenite - Alkalic, or alkali, syenites are a special group of rocks that are differentiated from other syenites by the fact that they contain a feldspathoid mineral, such as sodalite, leucite, or nepheline. They also contain significantly less quartz than other syenites. Relatively rare on a global scale, one of the best examples of an alkalic syenite can be found at Mont St. Hilaire in Quebec, Canada. The most studied alkalic syenite is nephelinesyenite, which is remarkable for its extensive variation in mineralogy, habit, and appearance.
Amphibolite - The key components of amphibolites, amphiboles, are a group of extremely diverse, widely prevalent rock-forming silicate minerals. In fact, their collective name stems form the Greek word amphibolo, meaning “ambiguous,” which French mineralogist René-Just Haüy, who coined the term in 1801, found indicative of the tremendous amount of variety displayed by members of the group. Amphiboles may, for instance, appear as prismatic, needle-like, or fibrous crystals and can display a tremendous array of colors, ranging from white, green, and blue to dark brown and black.
Amygdaloidal Basalt - The appearance and structure of basalts may vary significantly, though they are typically dark gray or black in color. Found worldwide amid the oceans as well as the continents, the rocks form when a certain type of magma cools and hardens. Sometimes the result is a smooth, glassy rock, while other samples may instead display a porphyritic structure. Basalts may also be spongy or amygdaloidal. This latter form of the rock occurs due to the deposition of secondary minerals, such as calcite or quartz, in gas bubbles that become trapped within the rock during the cooling process.
Anhydrite - Anhydrite does not form directly from geologic processes, but rather occurs as water evaporates from the mineral gypsum. Both minerals are composed of calcium sulfate, but gypsum’s water loss results in a reduced volume of the rock in which it is contained, sometimes leading to the formation of caverns or grottos. Common in evaporites, anhydrite frequently can be found capping salt domes, such as those found in Texas and Louisiana. The mineral crystals often appear white or gray in color, but may also exhibit a blue to violet hue, in which case they are frequently called angelite.
Anorthosite - Lunar anorthosite, which was first obtained by humans through the Apollo space missions, may appear in either a coarse or finely crystalline form. White in color and relatively reflective, large accumulations of anorthosite are what give the lunar highlands a light appearance to those viewing them from the Earth. These highlands are believed to represent the earliest crust of the moon, and anorthosite samples from them have been dated at more than 4 billion years old.
Aplite - Aplite is light in color and exhibits a granular texture, similar to that of sugar. The rock is generally found in small masses independent from other minerals. Aplite does, however, frequently occur with pegmatite, a similar rock that is more coarsely grained. The two igneous rocks may even form thin strata, or layers, within each other and are believed to have developed at the same point in history. A relatively rare gemstone called chrysoberyl can be found in aplites and pegmatites.
Arenaceous Shale - Aranaceous shale is a kind of shale that contains appreciable amounts of sand and may exhibit a wide variety of hues. For instance, an arenaceous shale deposit in Sweden is a greenish-gray color, while in Kansas occurrences of the rock are red, and in China examples have been found that are a purplish hue. The red and purple coloring of shales is typically an indication of the presence of hematite or limonite, and mineral constituents that contain large amounts of ferrous iron are usually responsible for green, black, and blue shales.
Arfvedsonite Granite - In alkali granites, arfvedsonite is a common constituent, as are other sodic-amphiboles and pyroxenes. The mineral, which was first discovered in Greenland and is named for Swedish chemist Johan Arfvedson, is an iron-rich sodium silicate. It is dark green or black in color and has a relative hardness of 6. Deposits of alkali granite can be found in various locales, such as New England and Norway, but are most extensive in Nigeria.
Arkose - Arkose is a coarse sandstone rich in feldspar that typically exhibits a pink, gray, or reddish hue. The substance closely resembles granite, the rock from whose disintegration it is commonly derived, in appearance and is frequently utilized as a building material. Quartz is the principal material in arkose, but feldspar composes at least 25 percent of the material, and calcite, iron oxide, and micas may be present as well. When exposed to dilute hydrochloric acid (a frequent method of quickly indicating rock types), arkose effervesces slightly.
Augen Gneiss - Gneisses are classified based upon a variety of characteristics, such as constituent minerals, parent materials, and chemical composition. Some, such as augen gneiss, are also described based upon their structure. This type of gneiss gains its name from the elliptic or lens-shaped form of many of its mineral grains. Indeed, the word augen is German for “eye,” a reference to the appearance of these readily visible components of metamorphic rocks.
Banded Sandstone - The expansive sandstone landscapes found in the western United States are some of the most breathtaking in the world. The impressive spiraling monoliths, jagged cliffs, and deep canyons in the region are awe inspiring in their magnificence, drawing tens of thousands of hikers and tourists to the area each year. Similar to a sunset, the colors found in these sandstone formations commonly feature deep reds, oranges, and yellows. In some locations, these colors alternate in beautiful banded patterns, which resulted from rapid changes in the surrounding environment.
Barite - Commonly colorless or white, barite may also be found in hues of reds, blues, yellows, and greens. The mineral varies in crystal habit as well, sometimes occurring in large, tubular forms, and other time in plates or concentric aggregates. In the latter instance, the crystal pattern is often likened to that of a flower, and when these formations exhibit a red tint they are often called “desert roses.”
Basalt - Basalts may vary greatly in composition and form. Some types, for instance, are compact and display a smooth, glassy surface, while others are porous or feature large, distinct crystals in a groundmass matrix. These conspicuous crystals most commonly consist of augite, feldspars, and olivine. Basalts also typically contain about 50 percent silica, as well as a relatively large amount of iron and magnesium. Large deposits of basalt are present in numerous locations around the world, including the mid-oceanic islands, Iceland, Germany, Turkey, Australia, India, Italy, and the United States.
Bauxite - Generally formed though the weathering of a variety of rocks, in addition to hydrous aluminum oxides, bauxite may contain quartz, kaolinite, magnetite, hematite, rutile, zircon, and a number of other minerals. Due to their wide array of possible compositions, as well as to differences in geologic history, bauxites vary significantly in structure and hue. They may, for instance, be hard or soft, porous or dense, stratified or mottled, and gray, brown, yellow, red, or pink in color.
Biotite Gneiss - Although a frequent component of metamorphic rocks, such as gneisses, as well as those of igneous origin, biotite is rarely found in sedimentary samples because the mineral has a tendency towards alteration when subjected to chemical weathering. Interestingly, when the mineral is only partially altered, it is sometimes mistaken for gold, its lightened color and flaky appearance being reminiscent of bits of the valuable metallic element. Typically, however, biotite’s dark color prohibits such a mistake.
Biotite Hornblende Granite - Not only is hornblende often found in granite, but also in syenite, gabbro, basalts, gneisses, schists, and a wide array of other rock types. The calcium-rich amphibole, which generally ranges in color from a pure green to greenish-black, usually develops as short, thick prismatic crystals. It may also occur, however, in granular or fibrous forms, sometimes in massive deposits. Also, a type of hornblende called edenite, in which iron oxides comprise less than 5 percent of the chemical makeup, appears in significantly lighter hues of gray or white.
Biotite in Granite - A variety of classes of granites have been established by geologists, most of which are differentiated by the relative coarseness of the grain or the amounts of the constituent minerals that are present. Granites are, however, chiefly composed of plagioclase and alkali feldspars. Other minor minerals that are commonly incorporated in the rocks include muscovite, pyroxenes, and biotite. Indeed, biotite is almost always found in granites of all types, though sometimes in negligible quantities.
Bituminous Coal - In the series of carbonaceous fuels that ranges from peat to anthracite, bituminous coal is of middle rank, containing intermediate amounts of fixed carbon, volatile material, and moisture in comparison with the other materials to which it is related. Bituminous coal also exhibits a relatively high heat value, which along with its abundance, is responsible for its common use in a number of important practical applications. The material is, for instance, utilized to generate steam in industrial plants and to produce coke for the smelting of iron ore, two functions that are central to modern civilization.
Bituminous Shale - Bituminous shale is an argillaceous shale that contains bitumen, a mixture of hydrocarbons that may be used as fuel, as a paving material, or in a number of other applications. This type of shale is often found accompanying coal and, in fact, grades into bituminous coal when carbonaceous material is present in large amounts. Humans have utilized bituminous shale at various periods throughout history, though at some times much more heavily than others.
Breccia Marble - The angular fragments found in breccia marble may be derived from a single rock or many and are often held together by a cementing material, such as silt, silica, iron oxides, or carbonate. In some types of the rock, the cementing material is associated with the clasts it contains, but this is not always the case. When the breccia forms after a landslide or mudslide, for instance, the fragmentary material may be deposited in an unrelated area where the cement forms from a substance not typically connected with the clasts.
Camptonite - Lamprophyres are named based upon their mineral content. Camptonite, sannaite, and monchiquite, are types of the rock in which the predominant mafic minerals are amphibole, augite, olivine, and biotite. Camptonite is distinguished form these closely related rocks, however, by the fact that it contains a greater amount of feldspar than feldspathoid minerals and more plagioclase than orthoclase feldspar.
Chalk - In nature, chalk usually occurs in extensive deposits, many of which were formed along the bottom of the oceans millions of years ago during the Cretaceous period. In fact the name for this epoch stems from creta, the Latin word for chalk. Deposits of chalk are primarily comprised of the calcareous shells of foraminifera, which are a type of marine protozoa, but also the shells of diatoms, radiolarians, and similar organisms, as well as sponge spicules.
Chlorite Schist - Chlorite schist is a type of schist that contains appreciable quantities of a chlorite. Chlorites are a group of pervasive silicate minerals that are usually produced as alteration products of other minerals. Thus, they are widespread in rocks formed through metamorphism. Chlorites are typically green in color and often bestow this hue to the rocks in which they are contained. Chlorite schists, therefore, are characteristically olive or jade-like in color and in some areas are known as greenschists.
Chocolate Marble - Despite its delicious sounding name, chocolate marble is not edible. The rock gains its confectionary-like moniker purely from its appearance, which is a characteristic deep brown. Two of the best-known occurrences of chocolate marble are found in North America, one of which is located in Tennessee and the other of which is located in Ontario, Canada. Attractive and fine-grained, the marble deposits found in these locations tend to be well layered, facilitating their natural breakage into slabs that are ideal for cutting into building stones.
Clay Ironstone - Clay ironstone is a fine-grained sedimentary rock frequently described as a concretionary form of siderite. Essentially, the rock is composed of the carbonate or oxide of iron and clay or sand. The color it displays is usually gray, but the outer layers of the material are often brown from oxidation. Relatively heavy, the rock is most often found in the form of nodules, though it also occurs in larger masses that may be flat, round, or lens-like in shape.
Clinochlore - Clinochlore comprises part of a mineral series with chamosite. Though the two are extremely similar, chamosite contains significantly more iron than clinochlore and is more darkly hued, as well as less transparent, than its counterpart. Kaemmererite, on the other hand, is a type of clinochlore that is rich in chromium. This mineral substance is known for the beautiful shades of lavender and crimson that is displays, which stand out vividly against the more common greens of most other chlorites.
Coquina - Coquina is a rare type of rock, few deposits of the limestone having been identified around the world. The best-known occurrences of coquina, however, can be found in Florida. In fact, after losing a number of wooden fortifications in fires, coquina became the preferred building material for Spaniards inhabiting St. Augustine, the first permanent European settlement in the United States, during the 1600s. Thus, fittingly, coquina is a Spanish term that derives from the word cockle, which is believed to be a diminutive of form of concha, meaning "shell."
Cordierite Anthophyllite Skarn - Cordierite is a blue magnesium aluminosilicate mineral that is sometimes utilized as a gemstone. Indeed, samples that have been procured from Sri Lanka are sometimes referred to as water sapphires. A synthetic variety of the mineral, however, which has a very low coefficient of thermal expansion and excellent resistance to thermal shock, is primarily used in items such as electrical insulators and high performance resistors.
Dacite - In magma form, dacite is extremely viscous, hindering it from moving very far away from its point of eruption before it cools and solidifies. The substance is, therefore, highly involved in the creation of thick volcanic domes. Dacite, which typically erupts at temperatures that range between 800 and 1000 degrees Celsius, is also one of the rocks most commonly associated with the large volcanic explosions called Plinian eruptions.
Diffusion Dolomite - Dolomite is a kind of carbonate rock that is similar to limestone, but is generally heavier and harder than that type of sedimentary rock. Due to their analogous nature, dolomite is often utilized in place of limestone in a variety of applications, especially as an aggregate in cement mixes. The rock also gains frequent use as a building stone, and when it undergoes metamorphism it becomes dolomitic marble, which is popularly utilized for ornamental purposes.
Diopside Gneiss - Diopside, the characteristic component of diopside gneiss, is an important rock-forming silicate mineral found in many metamorphic and igneous rocks, as well as meteorites. A member of the pyroxene family, diopside is part of a chemical replacement series that also includes hedenbergite and augite. Iron and magnesium ions can be freely substituted between these three minerals. Although it is frequently a lustrous white or green in color, diopside may exhibit a variety of hues and patterns.
Diorite - Diorite may be dark gray to black, sometimes with a greenish hue, depending on the percentage of dark minerals it contains. The rock is formed in the continental crust above subduction zones and is found worldwide, but primarily in the roots of mountains. Diorite may occur independently, but is often associated with granite and gabbro intrusions, with which the rock sometimes merges. Diorite is utilized as a building material and as an ornamental rock, though it is not particularly high in demand, perhaps because of its dark coloration.
Diorite Gneiss - Diorite gneiss chiefly consists of sodium-rich plagioclase feldspar, quartz, and hornblende, as well as minor amounts of biotite, sphene, or other accessory minerals. Depending on the exact mineral constituents and their relative abundance, the rock may exhibit an overall light or dark appearance, though all varieties exhibit the distinct banding effect characteristic of gneiss. When polished, diorite gneiss displays a good luster, resulting in its heavy use for sculptures in ancient Egypt, where significant quantities of the stone could be found.
Dolomite Marble - As indicated by its common name, dolomite marble is that which is composed primarily of the mineral dolomite. The crystalline substance, which may be gray, white, buff, bluish, reddish, or greenish in color and exhibits a pearly to subvitreous sheen, is quarried in a number of locations throughout the world, such as Finland, Wyoming, New York, Italy, and Greece. Large formations of dolomite are also found in the region of the Alps typically referred to today as the Dolomites, where the rock was first scientifically studied in late eighteenth century by Dolomieu.
Dunite - Originally found in the Dun Mountain of New Zealand, dunite was given its moniker by the geologist Christian Gottlieb von Hochstetter in 1859. The rock, which readily degrades into serpentine, is a particularly important commercial source of chromium, but may also contain copper, magnesium, and platinum ores. In fact, the desire to mine and transport the large amount of chromium present in the Dun Mountain led to the creation of New Zealand’s first railway system in 1862.
Eclogite - Eclogite is formed when igneous or metamorphic rocks that contain significant amounts of mafic minerals are exposed to pressures that exceed 1.5 gigapascals and high temperatures. Though chemically similar to basalt, the appearance of eclogite is quite distinct, being significantly more colorful than many other types of rocks. Thus, the coarse- to medium-grained green and red (sometimes pink) mineral matter is relatively easy to identify, even to those with an untrained eye.
Ferruginous Argillite - Argillite is primarily composed of clay or silt particles, but may contain other constituents as well. When iron is present, the rock is commonly referred to as a ferruginous argillite. This type of argillite may be found in various locales, such as Missoula, Montana and Killarney Park in Ontario, Canada. A siliceous kind of argillite, on the other hand, is commonly known as zebra rock due to the alternating red and cream-colored bands it exhibits.
Ferruginous Shale - Relatively porous and soft, shale is able to readily take up and release minerals and organic material. Thus, the composition of shales is quite various, heavily depending on the surrounding environment. Ferruginous shale is a type of shale that is rich in iron. The rock typically exhibits a reddish hue if the iron present is ferric or a green tint if the iron is ferrous. Dark gray or black shale, however, often results from the presence of significant amounts of organic matter.
Flint - Flint occurs in a variety of colors depending upon the impurities it contains, but is most often brown, dark gray, or black. A fairly common type of flint called chert, however, is usually much paler than other varieties. Flint may also exhibit a white coating if it is mined from chalk or other lime-containing deposits, and several varieties of the mineral take a good shine when polished. Thus, flint is sometimes utilized in jewelry and ornamental items.
Fuchsite - Fuchsite is not commonly utilized for commercial applications, but due to its beautiful coloring and glassy luster, the mineral is a component in several rocks that are polished and sold as gemstones. Greenlandite, for instance, is the nongeologic name of a green to bluish-green quartzite from Greenland that depends upon fuchsite for its color. Similarly, the substance marketed as Indian jade is often quartzite interspersed with green fuchsite flakes.
Gabbro - Darkly or greenish colored, gabbro is an intrusive rock formed when molten magma becomes trapped in cracks or other layers of rock and gradually cools into a rigid, crystalline mass. The mineral material occurs in large bodies and is common in various locations, such as South Africa, Sudbury, Ontario, Scotland’s Isle of Skye, and the Adirondack Mountains in New York. A relatively dense rock, gabbro is primarily composed of pyroxene and plagioclase feldspar, but olivine, amphibole, and quartz may also be present.
Garnet Wollastonite Skarn - The term skarn was originally coined in Sweden to refer to iron deposits that occur in rocks bordering an igneous intrusion, but today it is more generally used for pluton-neighboring rocks containing any type of ore. Among the minerals commonly found in skarns are garnets and calc-silicates, such as wollastonite. Wollastonite crystals typically appear glassy white, but may occasionally be found in shades of gray. Garnets may be found in a wide variety of colors, which contributes to their widespread popularity as gemstones.
Glauconite - The mineral glauconite, which characteristically appears some shade of green, derives its name from the Greek word glaukos, meaning “bluish-green.” Typically found in small grains referred to as pellets, the color of the substance is a consequence of its iron content and its other constituent elements. Glauconite can be precipitated directly from marine waters or may alternatively occur via a transformation of the organic wastes of bottom-dwellers or through the underwater modification of certain clays.
Glaucophane Schist - Glaucophane only occurs in certain rocks, such as schist, marble, and eclogite, and is generally formed in a highly metamorphic zone known to geologists by the term blueschist facies. The typical location of blueschist facies is along continental margins effected by shifting of oceanic plates and in areas of high volcanic and seismic activity. Some of the best-known examples of blueschist facies in the world are found in Japan, California, the Mediterranean, and the Alps.
Granitoid Gneiss - Granite is a type of visibly crystalline igneous rock primarily composed of quartz and feldspars that has been utilized as a building material since antiquity. Generally light in color, the rock also often contains minor amounts of hornblende, mica, or other accessory minerals. As its name implies, granitoid gneiss is a variety of gneiss that exhibits many of the properties characteristic of granite. Notable examples of granitoid gneiss can be found in Columbus, Georgia and in South Africa.
Granodiorite - An intrusive igneous rock, granodiorite often forms large masses in the roots of mountain ranges, such as the Andes. It also often occurs in association with granitic batholiths. Indeed, granodiorite is often confused with granite, a rock with which it shares a similar appearance, texture, and composition. Granodiorite does, however, typically appear somewhat darker than granite because it contains a greater amount of plagioclase feldspar. Other mineral components of granodiorite include quartz, orthoclase feldspar, and, to a lesser extent, hornblende, biotite, apatite, and sphene.
Graphitic Marble - Graphitic marble is a type of marble that contains appreciable amounts of graphite, a steel-gray to black mineral comprised of carbon. Opaque and very soft, graphite deposits in marble usually appear as gray or silver flakes or swirls when polished, giving a characteristic appearance to the rock. However, when present with other accessory minerals, the overall look of the marble may be quite different, sometimes exhibiting hues of blues, reds, greens, pinks, and yellows as well.
Gray Sandstone - In its purest form, sandstone is glassy and white in color, but the material is more typically found in shades of brown, yellow, red, and gray. The color of the rock is determined by the type of minerals it contains, as well as the characteristics of the material that bonds them together. The presence of iron oxide, for instance, results in a yellow, orange, or reddish-brown hue, while significant amounts of potassium feldspar impart a pinkish tint to the rock. The occurrence of gray sandstone is typically a sign that the rock contains calcite cement.
Graywacke - Graywacke formation is associated with landslides that occur underwater, which carry fragmented rock, sand, silt, and other debris away from the continental periphery and into the deep waters of the ocean. Indeed, this rock type is particularly linked to subduction zones, where the collision of the oceanic and continental lithospheres results in frequent sediment release. Since in such unstable locations minerals, rocks, and other particles do not have much time to sort themselves out before they accumulate, when graywacke is formed from the materials, they seem to be mixed haphazardly throughout the rock, which has resulted in its alternate name, dirty sandstone.
Green Slate - Slates exhibit perfect cleavage into thin, broad layers of rock due to the orientation of the flaky minerals they contain. Thus, these economically important rocks are readily quarried into smooth surfaced sheets of material that can be used for a variety of applications. Some of the earliest uses of slate include tombstones, tiling, and writing tablets, appropriately known as “slates.” Among the items that slate has more recently been used to make are blackboards, sinks, mantels, and pool tabletops.
Greenstone - Many of the oldest rocks on Earth are greenstones, and some of the oldest fossils ever identified may be found among them. In fact, during the 1990s, it was suggested that microscopic structures found in 3.465-billion-year-old Western Australian greenstones were the fossils of 11 different species of bacteria. If this proposal is true, the fossils in the greenstones formed more than a billion years before the oldest undisputed bacterial fossils. Such information would undeniably indicate that life on Earth developed much earlier than was previously supposed.
Gypsum - Gypsum can be found around the world in a variety of forms and is often known by different names based upon its appearance. When it occurs as clear, well-developed crystals, gypsum is commonly referred to as selenite. However, the fibrous form of the mineral, which has a characteristic luster, is known as satin spar. It is this form of gypsum that finds use in a wide variety of jewelry and ornamental work. Massive deposits of fine-grained gypsum, on the other hand, are most commonly called alabaster, a material frequently utilized to construct statues.
Hedenbergite Syenite - Although syenite is similar to granite in many regards, the rock contains little or no quartz. Indeed, essentially syenite is comprised of a ferromagnesian mineral and an alkali feldspar, although some varieties also contain a feldspathoid mineral. The exact type of these basic components may vary significantly, and many syenites are named or classified by the dominant mineral they contain. Hedenbergite syenite, for instance, is a type of syenite that contains significant amounts of hedenbergite, a member of the pyroxene family of minerals.
Hematite - Due to its profuseness and high iron content, hematite is considered one of the most important sources of iron ore. The mineral occurs, however, in a variety of forms, some of which are more commercially valuable than others. The type of hematite that occurs in large sedimentary deposits, such as the ones found near Lake Superior and Quebec in North America, is the most notable for its metal ore. Interestingly, according to ancient folklore, such sizable deposits of hematite were formed from the bloodshed of fierce battles seeping into the ground.
Hornblende Gneiss - The distinct banding that is apparent in gneisses is usually due to a segregation of light and dark colored minerals in the rock. The light bands within the rock primarily consist of quartz and feldspar, but the composition of the darker bands may differ significantly depending on the sample. A gneiss in which the dark green or black amphibole mineral hornblende is largely responsible for the dark coloration is typically referred to as a hornblende gneiss.
Hornblende Schist - The minerals that are found in schists vary significantly, but talc, chlorite, muscovite, biotite, and graphite are some of the more common constituents. In order to more explicitly identify samples of the rocks, schists are typically classified and described based upon their mineralogical composition. Hornblende schist, for example, is a variety of schist rich in the amphibole mineral hornblende, though the rock may also contain an abundance of plagioclase feldspar and other substances as well.
Hornblende Syenite - Hornblendes are a subgroup of amphibole minerals that contain significant amounts of calcium and are monoclinic. Quite common, hornblendes are associated with a wide variety of rocks, such as syenites, granites, gneisses, and gabbros. The minerals are typically opaque and dark green to black in color with a vitreous to dull luster. Also, hornblende crystals may be prismatic or long and thin, and they sometimes occur in massive, granular aggregates as well.
Ijolite - The crystalline rock ijolite is considered the plutonic analogue to olivine-poor nephelinites and nepheline dolerites. In addition to nepheline, it is chiefly composed of an alkali pyroxene mineral, but may also contain various other materials, such as calcite, sphene, melanite, wallastonite, and pectolite. Relatively rare, it may be found in Magnet Cove, Arkansas, Iron Hill, Colorado, the Kola Peninsula of Russia, and Canada’s Ice River, among other less well-known locales.
Kyanite Quartzite - Kyanite is an aluminum silicate mineral that occurs primarily in schists and gneisses, often in association with quartz, mica, or garnet. Similar to many other minerals, attractive specimens of kyanite are sometimes cut into gemstones. The substance is perhaps more commercially important, however, as a source of mullite, a raw material utilized in refractory ceramics applications.
Lamproite - Once believed to be present only in kimberlite, in 1979 commercial-grade diamonds were found in a deposit of olivine lamproite in Western Australia. Six years later, production began at the Argyle diamond pipe, and the mine has since become the single greatest producer of carats per year. The vast majority of the diamonds extracted from the mine are utilized for industrial purposes, but approximately 5 percent are suitable for use as good quality gemstones.
Latite Porphyry - Considered the extrusive equivalent of monzonite, latite is a member of the calc-alkaline magma series and is chemically intermediate to andesite and trachyte. Though quartz is generally absent from the rock, when it is notably present in the groundmass, latite is an intermediary of dacite. Extremely plentiful in the western half of North America, latite is generally light hued, exhibiting colors such as gray, pink, yellow, and white depending on its constituent minerals.
Lepidolite - Pink to purple in color with a vitreous to pearly luster, lepidolite can be quite a beautiful mineral. Thus, the mica is often prized as an attractive mineral specimen and is sometimes sold as a healing stone. Also, a rock composed of granular pink lepidolite and red to pink tourmaline is often polished and utilized for ornamental carvings. Nevertheless, the rare mica is most highly valued for the large quantities of lithium it frequently contains.
Leucite Nepheline Tephrite - Leucite-tephrite is a type of igneous rock in which olivine is absent and plagioclase feldspar is present. When nepheline, the most common feldspathoid mineral in the world, also occurs in the rock, the basalt is described as a leucite nepheline tephrite. Similar to leucite, nepheline is sometimes utilized for commercial purposes, primarily as a substitute for feldspar in the production of ceramic and glass items. Extremely rare, leucite nepheline tephrite, which may exhibit an aphanitic to porphyritic texture, is best known from Hamberg, Germany.
Malignite - Malignite is a type of gabbro that derives its name from the Maligne River in Ontario, Canada where the type occurrence can be found. The rock generally contains nepheline and orthoclase feldspar in approximately equal amounts, each comprising approximately 20 percent of the rock. About half of the rock is composed of clinopyroxene, and the remaining ten percent of the gabbro’s composition usually consists of small amounts of sphene, biotite, and apatite.
Margarite - Margarite is a mica that features calcium as the interlayer of cations that holds each sheet of the mineral together. The substitution of this element for those more commonly included in micas results in a less flexible mineral. Thus, although margarite still exhibits the perfect cleavage characteristic to most micas, the thin sheets it produces when cleaved are not as bendable as those of many other varieties. Margarite is, therefore, classified as one of the brittle micas.
Micaceous Sandstone - Micaceous sandstone, which is one of the primary types of sandstone, contains appreciable quantities of mica minerals. The inclusion of the platy micas in sandstone deposits is readily apparent to the naked eye, making identification of hand samples of the rock relatively simple. Micaceous sandstones are typically found among rivers and lakes, and although any mica may be present in the rocks, the most common seems to be the light-colored mineral muscovite.
Mica Schist - The most common schist that occurs on Earth is mica schist, which essentially consists of quartz and mica, but may also contain some feldspar. Similar to other schists, mica schist is metamorphic and flakes of its component minerals (usually muscovite or biotite) are visible to the unaided eye. Abundant amid Precambrian rocks, a variety of valuable materials may be interbedded with mica schists.
Micropegmatite Granite - When the unusual crystal structure of graphic granite occurs on a microscopic scale, the rock is usually referred to as micropegmatite, though it is also occasionally known by the name of microgranite. This structure is specifically comprised of patches of quartz crystals in parallel orientation, which usually appear triangular in cross section, and quartz-feldspar interfaces that are planar. The remarkable angularity and parallelism of the crystals of quartz are generally believed to be a reaction to the conditions present during their growth.
Monzonite - Quartz monzonite, which is present in many mountain belts, is extremely resistant to erosion. In fact, the hardness and durability of the substance has been centrally involved in the formation of many natural wonders, such as the awe-inspiring Curecanti Needle in the Black Canyon of Colorado. This towering spire was created over thousands of years, as the rushing waters of the Gunnison River and its tributaries slowly weathered away the softer surrounding metamorphic rock, leaving only the resilient quartz monzonite monolith in its path.
Muscovite - Typically transparent to translucent, although it may exhibit white, silver, brown, green, or yellow hues, the color of muscovite is dependent upon its iron content. Thin sheets of the mineral, which can be found worldwide, are quite flexible and were formerly used as windowpanes in the Muscovy region of Russia. This usage led to the material being commonly referred to as Muscovy glass, a term which is the origin of the modern name of the mineral.
Mylonite - Mylonites, which can be found among rocks of all ages, are typically believed to be only formed in shear zones. Pressure, temperature, and strain are the primary factors that determine whether or not a mylonite or some other type of metamorphic rock will form when intense folding or faulting occurs. If these dynamics fall outside of a certain range, the process of metamorphism may result, for instance, in a cataclastite, gneiss, or granulite, rather than a fine-grained, highly laminated mylonite.
Nepheline - Typically white, gray, or brownish in color, nepheline often exhibits a greasy appearance, but may also display a vitreous luster. The cleavage of nepheline is poor and when the mineral is placed in acid, it produces a hazy frost of silica gel. This characteristic is responsible for the name of the mineral, which is derived from the Greek word for “cloud.” Nepheline is sometimes confused with quartz because of their comparable optical properties, but the minerals do not appear together in the same rocks.
Norite - Norite typically occurs as discrete intrusive masses or as layers in large, basic intrusions of other materials, as do gabbros, and the two cannot generally be distinguished from one another without the aid of a microscope. A particularly significant intrusion of norite can be found in Ontario, Canada. In fact, a cavity more than 30 meters deep has been excavated from this gigantic mass of rock in order to house the Sudbury Neutrino Observatory, which was designed to facilitate the detection of the elusive neutrino particles emitted from the core of the sun and exploding stars.
Novaculite - Relatively rare, only rock formations found in Texas, Oklahoma, and Arkansas are usually described as novaculite. The term derives from the Latin novacula, meaning “sharp knife” or “razor stone.” Such etymological roots are indicative of the sharpness of the edges of the rock when it has been chipped. Indeed, it was for the razor-like spearheads, dart points, and arrowheads that could be formed from the material that Native Americans primarily utilized the rock.
Obsidian Snowflake - Obsidian snowflake is a black variety of obsidian that features inclusions of a white mineral, which appear similar to snowflakes when polished. This mineral has been identified as cristobalite, a stable form of silica, in many samples. In some areas, the radiating crystals are believed to be more reminiscent of flowers than snowflakes, resulting in the material’s alternate moniker, flowering obsidian. The most notable occurrence of the unusual black and white glass can be found near Milford, Utah.
Olivine Basalt Porphyry - Porphyritic basalts are relatively common and are found around the world. The phenocrysts contained in them vary somewhat, but are typically comprised of the silicate minerals pyroxene and olivine. Olivine is one of the most prolific substances found in the upper mantle of the Earth and has been found in meteorites as well. Typically green in color, perhaps the most familiar of these anisotropic minerals is peridot, a type of olivine that serves as the birthstone of August.
Olivine Pyroxene Andesite - Andesite is a type of finely crystalline rock of volcanic origin that typically consists of andesine or another plagioclase feldspar and one or more ferromagnesian minerals, such as pyroxene and olivine. This type of rock, which is common in the Andes Mountains from whence it gains its name, typically occurs in porphyries, large crystals of the feldspar or dark minerals it contains jutting out of a fine-grained matrix.
Orbicular Diorite - Orbicular diorite is an extremely rare variety of the diorite that contains many spherical lumps, or orbs, that exhibit concentric zones of light and dark color. Once only known on the island of Corsica, occurrences of orbicular diorite have now been found in South Africa, North Carolina, and a few other locales as well. Yet, how this unusual type of diorite forms has yet to be determined with certainty and is a matter of significant debate among geologists.
Peperino Tuff - Peperino tuff is best known as building stone heavily utilized by the ancient Romans. The rock is a special variety of trachyte tuff that contains little or no quartz, but significant amounts of orthoclase and oligoclase felspar with scattered crystals of biotite, augite, and various other dark minerals. Although peperino tuff is primarily gray or brown in color, the name of the rock refers to the dark inclusions of these minerals, which were thought to resemble peppercorns in the archetypal peperino found in Italy.
Peridotite - When weathered, peridotite often exhibits a brownish appearance due to the formation of iron oxides, but the rock is more familiar in its fresh, green form. The typical coloration of peridotite is indicative of its mineralogical content, which is comprised of at least 10 percent of the yellowish-green mineral olivine. Peridotite also usually contains significant amounts of pyroxenes and other iron- and magnesia-rich minerals, but less than 10 percent feldspar.
Phlogopite - A hydrous silicate of potassium, magnesium, and aluminum, phlogopite often exhibits an unusual copperlike or bronze-red color, though it may also be yellowish or greenish hued. The precise coloration of the substance primarily depends upon its iron content, which also affects the density of the mineral. Phlogopite is usually formed metamorphically, but may also develop from plutonic forces.
Phosphorite - Often alternatively referred to as phosphate rock, phosphorite is frequently associated with chert, shale, and sandstone. The phosphate that is contained in the rock is typically present in dense masses or nodules, which are the chief global resource exploited for phosphorous-containing fertilizers. Deposits of phosphorite can be found in various locations around the world, but the nation leading in both production and consumption of the rock is the United States.
Pink Marble - The marble most popular among ancient Greek sculptors and many subsequent artists, such as the famous Michelangelo, is pure white in color. However, marble may also display a variety of color variegations that can impart a pink, yellow, gray, green, blue, red, black, or buff hue to the material. These characteristic swirls or bands of color, which appear from impurities in the rock, are usually believed to only add to the beauty of marble. Thus, numerous buildings and ornamental structures also utilize these varieties of the rock, although typically indoors because marble can be readily corroded by the elements and impurities further diminish its durability.
Potosi Dolomite - The Potosi Dolomite gains its moniker from Potosi, Missouri, where the large formation of cherty carbonate rocks was first identified in the 1800s. From exposures at its type locality, geologists traced the rock mass beneath the surface of the Earth to exposures of the material found in northern Illinois. In that locale, the Potosi Dolomite is sometimes alternatively referred to as the Trempealeau Formation. The term Potosi Dolomite is also frequently utilized in regard rocks found in Indiana.
Pyroxenite - Pyroxenite is a type of intrusive igneous rock that is predominantly composed of pyroxenes. Relatively rare, when pyroxenite does occur it is typically found in dark-colored layers, sheets, veins, narrow dikes, and similar discrete forms. Some of the more notable occurrences of pyroxenite are located in Canada, Ireland, South Africa, and the Pyrenees mountain range. Most of the pyroxenite rocks found in these locations and elsewhere have been named for the chief pyroxene they contain.
Quartz Conglomerate - Quartz conglomerate is a lithified sedimentary rock that is primarily composed of rounded pebbles of quartz. These pebbles are generally greater than two millimeters in diameter and are cemented together in such away that the rock appears quite similar to concrete, with which it is often confused by casual observers. One of the largest quartz conglomerate deposits in the world can be found in Olean, New York.
Quartz Monzonite Porphyry - Quartz monzonite is a granitic rock that is primarily composed of quartz, plagioclase feldspar, and orthoclase feldspar. Other minerals, such as biotite and hornblende, are also typically present in small amounts. Abundant and widespread, the intrusive igneous rock may be found in tremendous masses in many mountainous areas. Quartz monzonite is also frequently involved in gorge formation since it is extremely resistant to weathering and stands tall as surrounding rocks erode away.
Quartz-Sericite Schist - Typically schists contain significantly lower levels of quartz than gneisses and many other rocks, but quartz-sericite schist is a variety of the rock than contains a greater amount of the mineral than normal. As its name implies, this type of schist also contains significant amounts of sericite, which is a fine-grained type of either muscovite or paragonite, both of which are relatively abundant silicate minerals. Found in various locations, one of the best known examples of quartz-sericite schist occurs in Canada, where it is a source of economically important gold deposits.
Quartzite Jasper Pebble - Jasper is a colored variety of chalcedony, a type of quartz with a cryptocrystalline structure that can only be seen with the help of a microscope. The beautiful mineral, which is opaque, may appear in a variety of hues including pink, yellow, green, blue, and gray, but is perhaps best known in a shade of reddish brown. Impurities in jasper may create a variety of banding patterns in the stone as well, some of which are said to resemble landscapes.
Red Sandstone - Red sandstone, which is especially known for its great beauty in the western United States, often forms spectacular vistas and natural structures that serve as tourist attractions. Large red sandstone monoliths that jut out from the Earth towards the sky, for example, can be seen at various national landmarks and parks, such as the Garden of the Gods, a 1,350-acre park located in Colorado Springs, Colorado.
Red Slate - Slates are metamorphic rocks, most of which began as clay sediments deposited by water. The age of slates varies somewhat, but many were formed among old deposits of rock found deep beneath the surface of the Earth. Slate appears in a wide range of colors and greatly depends upon the mineral composition of the rock. Red slate, for instance, gains its hue from the significant amount of hematite, a crystalline form of ferric oxide, it contains.
Rhyolite Flow - Rhyolite is an extrusive igneous rock that is usually light in color, often exhibiting, white, gray, brownish, magenta, or reddish hues. Samples may also be multicolored or exhibit distinct banding. Certain varieties of rhyolite are utilized as gemrocks or for ornamental purposes. A thinly banded type is commonly known as wonderstone, although the same name is sometimes applied to a similar looking variety of banded sandstone. Some spherulitic rhyolites are known commercially as birdseye, and other ornamental types of rhyolite have been known by the names elixerite, hickoryite, and liparite.
Rhyolite Porphyry - Relatively abundant around the world, rhyolites have been formed in all geologic ages. Most of the rocks are porphyritic in texture, consisting of a fine-grained base through which sizable crystals, called phenocrysts, are dispersed. The two different sizes of crystals in these rocks is an indication that the rhyolite cooled in two separate stages. Crystallization that begins when the magma is still buried deep within the Earth produces the phenocrysts, while the groundmass they are lodged in does not form until after an eruption brings the magma to the Earth’s surface.
Sericite - An appellation that applies to more than one substance, sericite is typically defined as a fine-grained type of either of the minerals muscovite or paragonite. The substance typically occurs as aggregates of tiny flakes and exhibits a silky sheen, which it imparts to the rocks that it is found in. The muscovite-based type of sericite most often occurs in phyllites, schists, and other rocks that exhibit a fine-grained, layered structure.
Serpentinite - Associated with subduction zones, serpentinite is a metamorphic rock that is formed from the action of high pressure and heat upon hornblende schists or igneous rocks composed chiefly of mafic minerals, such as peridotite, gabbro, or basalt. The substance is most often found in areas where mountain ranges have formed due to the sealing off of an ocean basin. For instance, the Coast Ranges in California, where serpentinite is the official state rock, primarily consist of slices of the ocean’s crust that have been faulted and folded along the coastline.
Shonkinite - Many early geologists named rocks that were readily distinguishable from other types of rocks as soon as they were discovered. Often times these names were based upon the place where the rocks were first identified. Indeed, the term shonkinite derives from Shonkin, the Native American name for Montana’s Highwood Mountains, where this rare type of rock was first examined by Walter Harvey Weed and Louis V. Pirsson in 1895.
Sillimanite Garnet Gneiss - Sillimanite is an aluminum silicate that usually occurs in rocks that are rich in clay and have undergone regional metamorphism at high temperatures, such as gneisses. The mineral varies in color, but often appears glassy white, green, or brown. There also exists, however, a light blue form of sillimanite that can be found in Sri Lanka, which is sometimes utilized as a gemstone. Typically long and slender in shape, crystals of sillimanite frequently occur in fibrous masses, but are generally too brittle to be considered asbestos.
Soapstone - Widely utilized for thousands of years, soapstone is a relatively soft rock that can be readily carved into an array of items with a knife or other sharp object. It has been used since antiquity, for instance, to create sculptures, jewelry, and other ornaments, as well as more practical items, such as cooking utensils. In colonial America, soapstone gained significant use as a building material and became a particularly popular choice for sinks and fireplace hearths.
Sodalite Syenite - Syenites are often named based upon the chief ferromagnesian mineral they contain. Thus, sodalite syenite is a type of syenite that contains appreciable amounts of sodalite, a sodium aluminosilicate mineral. This variety of rock may also include a wide array of other minerals, such as granconite, rutile, pyrite, and gypsum, but in smaller amounts. Relatively rare, sodalite syenite is typically lighter in color than more common varieties of the rock, generally exhibiting a pale gray to beige hue.
Staurolite Quartzite - Common in metamorphic rocks, such as quartzite, staurolite often forms in such a way that its prismatic crystals are twinned, giving them the appearance of a cross. Due to this unusual manifestation, the mineral has been revered as a good luck charm since antiquity when it was referred to as lapis crucifer, or cross stone. Some also call samples of the mineral fairy crosses, and, according to one legend, the tears of fairies that mourned the crucifixion of Christ formed the crosses.
Tactite Skarn - Skarns can be found around the world and most developed during the Mesozoic period some 63 million to 230 million years ago. Zones of skarn are usually relatively irregular in shape due to the erratic characteristics of the host rocks, which vary in porosity, permeability, and lithology. The zones often contain deposits of iron, copper, gold, magnetite, tungsten, and other ores, though in relatively small amounts. Nevertheless, the small deposits are often mined for commercial profit.
Talc-Tremolite Schist - Talc is a mineral that is most commonly known in the form of soft white talcum powder. However, the substance, which is usually described as exhibiting a greasy, soapy feel, may occur in a range of colors that extends from gray to red to brown, depending on the impurities present. The hydrous silicate of magnesium, which sometimes occurs as an alternation product of tremolite, may also be found in a variety of forms, including finely or coarsely granular masses, as well as the thin layers known as folia in which it most commonly occurs.
Tholeiitic Basalt - Two major groups of basalts are generally scientifically recognized: tholeiitic basalts and alkali olivine basalts. Tholeiitic basalts are formed from magmas of the same name, which produced the mid-ocean ridge basalt and built the Hawaiian Islands, Iceland, and other mid-oceanic landmasses. This type of basalt is present on Earth in much greater amounts than alkali olivine basalt, which occurs along continental rifts and on oceanic islands.
Tonalite - Sometimes alternatively referred to as quartz diorite, tonalite is typically comprised of plagioclase feldpar, alkali feldspar, and more than 20 percent quartz. In addition, accessory minerals that are often present in the rock include pyroxenes and amphiboles. Coarse-grained, tonalite is somewhat like granite in appearance, but is often darker in color. Some specimens, however, exhibit what is usually termed "salt and pepper" coloration, consisting of a black and white dappled pattern.
Trachyte Porphyry - First discovered in ancient Thrace, the name for trachyte is generally believed to have been derived from a Greek word meaning “rough” or “coarse.” Such etymology is quite fitting since trachyte typically occurs as porphyry, a rock in which large, conspicuous crystals are set in a fine-grained matrix. Trachyte also often features a banded or striated appearance due to the flow of the lava from which it formed. The color of the rock varies, but the hues it exhibits are usually light and muted.
Travertine - The term travertine derives from an old Roman name for Tivoli, an Italian town where large deposits of the calcium carbonate can be found. Travertine quarried in Tivoli under the late Roman republic as well as the early empire was an extremely popular building material. It was, for instance, utilized to create the exterior of the famous Coliseum in Rome. Significant deposits of travertine also occur at Mammoth Hot Springs in Yellowstone National Park, Wyoming and various other locations around the world where limestone is common and calcium carbonate is present in circulating ground water.
Tuff - Due to the wide variance of characteristics that are associated with tuffs, the rocks are often classified in a number of ways in order to more specifically identify the material. Tuffs may, for instance, be described as crystal, lithic, or vitric, depending on the general nature of the material of which they are primarily comprised. The rocks are also sometimes described more specifically based upon the type of lava from which they formed or a specific substance which dominates their makeup.
Unakite - Unakite is a popular semiprecious gemstone often described as an epidotized granite. It usually appears as a mottled red or salmon pink and green stone. The green hue is attributed to the significant presence of epidote, a yellow-green silicate mineral that is sometimes produced during metamorphism of basic igneous rocks. The pink or red coloration, on the other hand, generally stems from pink orthoclase feldspar and quartz. The unique appearance of the highly polishable rock makes it an interesting choice for beads, jewelry, paperweights, and other ornamental items.
Variegated Dolomite - Dolomite was first studied by French mineralogist Deodat de Dolomieu, for whom the rock is named. Found around the world, formations of dolomite are relatively common and are often expansive in size. Indeed, dolomite is the dominant material in an entire region of the Alps, which is now appropriately known as the Dolomites. Other notable occurrences of the rock are located in the United States, Brazil, Africa, and Mexico.
Verde Antique - Verde antique is usually green in color, but may also display a yellowish hue. It is primarily utilized as an ornamental stone and was particularly popular among the ancient Romans, who often used it to accent pillars and create ornate indoor decorations. Consequently, many buildings from the Italian Renaissance, as well as modern structures built in a Romanesque style, also utilize verde antique. In the United States, verde antique is quarried in several locations, including Georgia, California, New York, Massachusetts, Maryland, and Vermont.
Volcanic Ash - Volcanic ash is quite different from more familiar types of ash, such as that generated by burning paper or wood. Instead of being soft or fluffy, the pyroclastic material is hard and insoluble in water. Particles of volcanic ash may also be much smaller than other ashes, often exhibiting diameters less than a thousandth of an inch in size. The mineralogical composition of volcanic ash generally depends upon the type of volcano it originates from and whether or not the material it ejects is primarily acidic or basic in nature.
Volcanic Sandstone - Abundant around the world, shale is the only sedimentary rock more common on Earth than sandstone. This fact, along with the diverse textures and mineral compositions the rock may exhibit, makes sandstone a valuable tool for geologists trying to achieve a better understanding of the erosional or depositional actions that have taken place in specific locales. Sandstones are perhaps more broadly utilized, however, for construction purposes. When simply quarried and cut, they make excellent building stones, and when crushed, sandstone may be used in a manner similar to common sand.
Welded Tuff - Welded tuff is significantly harder than other types of tuff and can be found in various locations around the world. Also known by the term ignimbrite, the rock covers vast expanses of land in Guatemala, New Zealand, Peru, and parts of the United States. One of the most famous occurrences of welded tuff can be seen at Yellowstone National Park in Wyoming. Created by a cataclysmic eruption more than 600,000 years ago, the huge welded tuff formation at the park is known as the Lava Creek Tuff.
White Marble - The most highly valued marble in ancient times was pure white in color. The beautiful material, which could be found in several locations in Greece, was heavily quarried and exported to many other areas, most notably Rome. Numerous examples of ancient white marble sculptures can still be enjoyed around the world in museums, but these works of art appear quite differently than they did to their contemporary viewers. This is primarily because ancient Greeks and Romans painted their statues in brilliant hues and decorated them with various types of ornamentation.
John D. Griffin, Shannon H. Neaves, Nathan S. Claxton, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
Questions or comments? Send us an email.
© 1998-2015 by Michael W. Davidson and The Florida State University. All Rights Reserved. No images, graphics, scripts, or applets may be reproduced or used in any manner without permission from the copyright holders. Use of this website means you agree to all of the Legal Terms and Conditions set forth by the owners.
This website is maintained by our