The hardest naturally occurring substance, diamonds are perhaps the most cherished of all gemstones. Like graphite, they are a crystalline form of carbon, the chemical element that provides the basis for all life on Earth. The dense, strongly bonded crystal structure is responsible for the extreme hardness of this mineral, whose name derives from the Greek word adamas, meaning unconquerable or invincible.

Exactly how diamonds are formed is a matter of debate amongst geologists, but they do know that conditions of both tremendous heat and pressure are necessary. Geological evidence suggests that diamonds were formed millions of years ago in molten rock or magma far below the surface of the earth, where extreme conditions are known to exist. Over the years, diamond-bearing materials were pushed up into the crust, forming the funnel-shaped kimberlite "pipes" characteristic of many diamond deposits. Diamonds that have been eroded from old kimberlite deposits also occur as small rocks and pebbles in alluvial and glacial deposits.

Tiny opaque diamonds have also been found in certain types of meteorites and are believed to have formed from the high temperature and pressure of impact. Although their physical properties are identical to those of terrestrial diamonds, impact diamonds have a different crystalline structure. Microscopic diamonds are thought to be fairly abundant in outer space, the by-products of exploding stars, or supernovas.

On planet Earth, these prized gems were first discovered in river gravel between 800 and 600 BC in India, which was the only known source of diamonds until the eighteenth century. Since then, major deposits have been unearthed in Brazil, South Africa, Australia, and, more recently, Canada. In 1991, kimberlite pipes were discovered in the Arctic region of Canada's Northwest Territories. The gems are now are being mined and sold under the trademark name of Polar Bear diamonds. Each diamond carries a laser-engraved polar bear logo, visible under a 10x power magnifier or stereo microscope.

Diamonds are heavily used not only as jewelry, but for a variety of industrial applications, lapidary work, and for the cutting edges of drills, saw blades, milling heads and other cutting tools. Synthetic diamonds have been produced commercially since 1960, but primarily as grit and small crystals for industrial purposes. Gem quality diamonds are too expensive to synthesize.

Many gem scholars agree that the tradition of birthstones arose from the Breastplate of Aaron described in the Bible (Exodus 28, 15-30). The breastplate was a ceremonial religious garment set with twelve gemstones that represented the twelve tribes of Israel and corresponded with the twelve signs of the zodiac and the twelve months of the year. There are many different birthstone lists, however, and some argue that they should be assigned by astrological sign and not month. Diamond is the birthstone for April and the zodiac sign Aries.