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A gemstone may be a pure chemical element (diamond is essentially pure carbon), a relatively simple chemical compound (quartz is silicon dioxide, SiO2), or a more complex mixture of various compounds and elements (the garnet family includes a highly variable mix of iron, magnesium, aluminum, and calcium silicates). The great majority of familiar gem materials are oxides or silicates (i.e., they contain oxygen and perhaps silicon) and formed as crystals during the cooling of the earth's crust over past millenia.
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