Smithsonian Handbooks: Gemstones

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From Wikipedia: A gemstone or gem (also called a precious or semi-precious stone, a fine gem, or jewel) is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments.[1][2] However certain rocks, (such as lapis lazuli) and organic materials (such as amber or jet) are not minerals, but are still used for jewelry, and are therefore often considered to be gemstones as well. Most gemstones are hard, but some soft minerals are used in jewelry because of their luster or other physical properties that have aesthetic value. Rarity is another characteristic that lends value to a gemstone. Apart from jewelry, from earliest antiquity until the 19th century engraved gems and hardstone carvings such as cups were major luxury art forms; the carvings of Carl Fabergé were the last significant works in this tradition. ~~~ The traditional classification in the West, which goes back to the Ancient Greeks, begins with a distinction between precious and semi-precious stones; similar distinctions are made in other cultures. In modern usage the precious stones are diamond, ruby, sapphire and emerald, with all other gemstones being semi-precious.[3] 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,[4] with hardnesses of 8-10 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.[5] 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...