Gems, also called gemstones or jewels, are a piece of mineral (or in the case of lapis lazuli a rock, or in the case of jet, amber and pearl organic material) in a cut and polished form. Gemstones are prized for their beauty, brilliance, color and rarity and often used in making jewelry. The distinguishing between precious and semi-precious gems is a relatively new practice; the term semi-precious to describe gemstones is thought to have originated in 1858. Before that time emerald, sapphire, and ruby were the dominant gems in the jewelry market, and when the market for other colored gemstones increased, the need for a term to differentiate them from emeralds, rubies and sapphires arose.
Diamonds, emeralds, sapphires and rubies are currently considered precious gems, and semi-precious gems include amethyst, tourmaline, peridot, onyx, garnet, opal and aquamarine. Diamonds, however, were not always considered precious stones. They actually became classified as precious after diamond mines were discovered in Africa in 1870, creating a large influx of diamonds on the market where demand was relatively low. This led to the establishment of the DeBeers cartel, a powerful entity which controls the worldwide supply of diamonds. Amethysts, on the other hand, were considered precious for hundreds of years after first being prized by the Ancient Greeks. Amethysts were reclassified as semi-precious when large deposits of them were discovered in South America in the early 1950's.
Precious gems often command a higher price in the market than semi-precious gems, although this is not always the case--and in fact the gem industry is attempting to do away with the term semi-precious. Diamonds and sapphires are frequently less expensive than garnets, tanzanite, and tourmaline; even aquamarine in large sizes can be more valuable than precious stones.
Gemstones are usually cut in two ways; cabochons or facets. Cabochons are round, smooth-cut gems usually of the opaque variety--opals, turquoise, jet. Faceting is used on translucent gems to increase the amount of light reflected from the gem. Facets are flat cuts made at angles all over the surface of the gem to maximize brilliance. Common faceted cuts include the emerald cut, princess cut, round brilliant cut and brilliant cut.