Emerald is a green variation of the mineral beryl that has been highly valued as a gemstone for thousands of years. Scholars speculate that the name derives from the Greek word smaragdos, a term that was also used when referring to other green-colored stones. The name emerald is also applied to other green minerals that are used as gemstones, such as tourmaline, hiddenite, corundum, and andradite.

Beryl crystals are some of the most well-known and prized gems that occur in nature, and include aquamarine, heliodor, and morganite, as well as emerald. Before 1925, gemstones were the primary use for beryl. Since then, beryl has become an important source of the element beryllium, formerly known as glucinium, a metal with a variety of modern commercial uses, particularly in manufacturing metal alloys.

The crystal structure of beryl emeralds is hexagonal (six-sided) with a hardness slightly higher than quartz but considerably less than that of diamond. The rich green color is created by small amounts of chromium or vanadium imbedded within the crystalline lattice. Emeralds are notorious for containing flaws, and flawless stones are rare and greatly valued.

The world's finest emeralds originate in the Muzo mining district of Colombia, where most are mined today. A rare stone, found only in this region, is known as a "Trapiche emerald", and is characterized by star-shaped rays that emanate from its center in a hexagonal pattern called an asterism, reminiscent of a star sapphire. Unlike star sapphires, ray patterns in emerald are not caused by light reflections from tiny parallel inclusions, but by black carbon impurities that happen to form in the same pattern. Emeralds have also been found in Australia, the Urals, Norway, Austria, and North Carolina. In the ancient Old World, emeralds were mined primarily in Egypt. Synthetic emeralds have been produced successfully since the mid-1930s. These crystals are very similar to naturally occurring crystals, evidencing the same color and beauty.

Many virtues and healing properties have been ascribed to emeralds over the years, when worn as a charm, from preventing epileptic seizures, curing dysentery, and driving away evil spirits to preserving the chastity of the wearer. The gems were even administered internally for their medicinal value.

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. Emerald is the birthstone for May and the zodiac sign Taurus.