Many gems are fashioned into rings or other jewelry.
A gem is defined by the Merriam-Webster dictionary as being a mineral or petrified material that, when cut and polished, can be used in jewelry. In the jewelry trade, gems or gemstones are divided into two categories: precious and semiprecious stones. Another type of stone that is used in jewelry is the organic gemstone. Although most gems are hard, because jewelry requires the stones to be durable, other gemstones are soft; their luster or other properties give them aesthetic value enabling them to be classified as gems.
The Diamond
The diamond can be put into a category of its own because it is the only gem that consists of the atoms from a single element, carbon. It is the hardest known naturally occurring substance on Earth, and is a type of crystallized pure carbon. Diamonds do not have to be colorless, and have been found in every color between colorless and black. Blue and blue-white diamonds are the most prized by connoisseurs; yellow and brown are common; and green and red diamonds are called "fancy" in the trade. The world's best diamonds are found in South Africa and Russia.
Emerald, Ruby and Sapphire
Other than diamonds, emeralds, rubies and sapphires are classified as being precious gems. Emeralds are a rich green variety of the mineral beryl, or beryllium aluminum silicate. The finest quality gems are mined in Colombia, though the world's largest emerald was found on a farm in North Carolina. Rubies and sapphires are chemically identical forms of corundum, or aluminum oxide. The different colors are caused by trace elements; in the case of the ruby this is chromium. Stones that are colored red are called rubies, and the best examples are found in Burma. All other colors are called sapphires, and although the color blue is usually associated with the sapphire, they can be any color other than red.
Semiprecious Gems
Gemstones other than those classified as precious fall under the category of semiprecious. The most recognizable names are amethyst, agate, beryl, jasper, citrine, garnet, obsidian, onyx, opal, topaz, tourmaline and turquoise, although there are many others. Semiprecious gems are all compounds that are found embedded in metamorphic rocks and that have been formed over millions of years. They can be either cut and polished or, in the case of opaque gems, fashioned and polished into cabochons: convex, rounded stones.
Organic Gems
Three types of gems can be classified as being organic gems, simply because they were each once part of a living organism. These are amber, coral and pearl. Amber is tree resin that has fossilized over thousands of years. Occasionally it can be found with an insect such as a mosquito embedded inside it, preserved for eternity. Coral has been harvested and used as jewelry for thousands of years, as have pearls.
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