Where Are Precious Stones Found?
Precious Stones, or gems, are minerals used for adornment, and they are found in rocks. Rocks are divided into three groups. The igneous (fire-formed) rocks may be fine-grained or coarse-grained; a very coarse-grained type, called pegmatite, is an important source of gem minerals such as diamonds. Gems are also found in the cavities of the igneous rocks granite and obsidian.
Sedimentary rocks are layered rocks and, except for turquoise and opal, are the sources of very few gems. However, when the original rock contained heavy minerals—and gem minerals are heavy—lumps of them tended to be deposited as pebbles in a river bed and such deposits from the gem gravels of Myanmar, the “byon”, and those of Sri Lanka, the “illam”.
Metamorphic rocks—rocks which have been altered by pressure—are a fruitful source of gem minerals, for instance the rubies found in Myanmar. There are also precious materials of animal origin-pearls from oysters, ivory from elephants and coral from the tiny sea creatures which give their name to it. There are also amber and jet, whose origin is vegetable. Amber is the fossilized resin of a coniferous tree which grew in the Eocene period. Jet is a variety of fossil wood.
Gems are found throughout the world and are prized for their rarity and beauty. Their charm may depend on transparency and depth of color as in the ruby and emerald, on color only as in the turquoise, on purity and “fire” as in the diamond, and on “play of color” as in the opal. The traditional classification in the West, which goes back to the ancient Greeks, begins with a distinction between precious and semi-precious; similar distinctions are made in other cultures.
In modern use the precious stones are diamond, ruby, sapphire and emerald, with all other gemstones being semi-precious. This distinction reflects the rarity of the respective stones in ancient times, as well as their quality: all are translucent with fine color in their purest forms, except for the colorless diamond, and very hard, with hardnesses of 8 to 10 on the Mohs scale.
Other stones are classified by their color, translucency and hardness. The traditional distinction does not necessarily reflect modern values, for example, while garnets are relatively inexpensive, a green garnet called tsavorite can be far more valuable than a mid-quality emerald.
Another unscientific term for semi-precious gemstones used in art history and archaeology is hardstone. Use of the terms ‘precious’ and ‘semi-precious’ in a commercial context is, arguably, misleading in that it deceptively implies certain stones are intrinsically more valuable than others, which is not necessarily the case.