Apatite
This interesting mineral is found occasionally in attractive green, blue, or violet stones, but is unfortunately too soft for extensive use in jewellery, the hardness being only 5 on Mohs’s scale. In composition it is a fluo-chloro-phosphate of calcium, corresponding to the formula Ca4[Ca(F,Cl)](PO4)3. When pure, it is devoid of colour, the tints being due to the presence of small amounts of tinctorial agents. The double refraction is uniaxial in character and negative in sign, the ordinary index being 1·642 and the extraordinary 1·646; the specific gravity varies from 3·17 to 3·23. The dichroism is usually feeble, but sometimes is strong; for instance, in the stones from the Burma ruby mines (yellow, blue-green). A cut stone might be mistaken for tourmaline, but is distinguished by its softness, or, when tested on the refractometer, by its inferior double refraction. It received its name from ἀπατάειν, deceive, because it was wrongly assigned to at least half a dozen different species in early days. Moroxite is a name sometimes given to blue-green apatite.
Beautiful violet stones are found at Ehrenfriedersdorf, Saxony; Schlaggenwald, Bohemia; and Mount Apatite, Auburn, Androscoggin County, Maine, U.S.A.; and blue stones come from Ceylon.