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Precious Stones
Exactly what constitutes a precious gemstone varies depending on factors such as rarity and fashion. Precious stones are commonly defined to include diamonds, emeralds, sapphires and rubies. Diamonds are formed from a very pure form of carbon. They can be colorless but certain impurities can create colored diamonds. Emeralds are a green form of the mineral beryl. Sapphires and rubies are formed from corundum, with trace amounts of other elements that give them their vibrant colors.
Semi-Precious Stones
Semi-precious stones are stones which are less hard, less transparent, or more common than precious stones. Gems made from these stones are sometimes more fragile or prone to damage from wear. Popular semi-precious stones include amethysts, garnets, agates, peridot, tourmaline, turquoise and moonstones. The designation "semi-precious" does not mean that these stones are necessarily less beautiful than precious stones--it's a matter of personal opinion.
Organic Materials
Although they are not rocks or minerals, certain organic materials are sometimes used to make jewelry, especially jet, amber, pearls, and coral. Jet, a black gemstone, is formed from fossilized wood. Amber is another fossilized material: tree resin. Amber often contains small inclusions of ancient plant matter; in rare cases, an entire insect is encased. Pearls are formed by oysters and some other mollusks. Coral is the remains of a colony of tiny sea creatures.
Synthetic Stones
Synthetic gemstones are chemically and visually identical to natural gemstones, but are created in a laboratory. Extreme heat and pressure are used to replicate the conditions that form natural gemstones. Among the many gems that can be created synthetically are diamonds, sapphires, and emeralds. Synthetic gems are considered to be worth less than natural gems.
Other Synthetic Gems
Some gems are completely man-made and are structurally different than natural gemstones, but designed to look like their natural counterparts. The cubic zirconia, a substitute for diamond, is probably the most well-known. Inexpensive stones can also be dyed and treated to look like precious gems. Paste gems, often used in vintage costume jewelry, are made from glass. Quality lead crystal glass gems can be attractive in their own right, however. Faux coral is made from dyed wood; faux amber from hardened---not truly fossilized---tree resin. Other faux gems are also made from acrylics and plastics.