The chemical composition of apatite is Ca5(PO4)3X, where X = F (fluorapatite, the most common), Cl (chlorapatite), OH (hydroxyapatite), or a mixture of all three; it forms prismatic or tabular hexagonal crystals, and also occurs as granular aggregates or in massive crusts. It is usually too soft for cutting and polishing, though there are two gem varieties. Hardness 5, specific gravity 3.1–3.4.
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