PREHNITE This species, which is named after its discoverer, Colonel Prehn, is found in nodular, yellow and oil-green stones, of which the latter have very occasionally been cut. It is a little soft, the hardness being only 6 on Mohs’s scale. The double refraction is large in amount, 0·033, biaxial in character, and positive in sign, the least and the greatest of the refractive indices being 1·616 and 1·649 respectively; the specific gravity varies from 2·81 to 2·95. In composition prehnite is a silicate of aluminium and calcium corresponding to the formula H_{2}Ca_{2}Al_{2}(SiO_{4})_{3}. The best material has been found at St. Cristophe, Bourg d’Oisans, Dauphiné, France. 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 Ca_{4}[Ca(F,Cl)](PO_{4})_{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. DIOPTASE Though of a pretty, emerald-green colour, dioptase has never been found in large enough crystals for gem purposes, and it is, moreover, rather soft, the hardness being only 5 on Mohs’s scale, and has an easy cleavage. In composition it is a hydrous silicate of copper corresponding to the formula CuH_{2}SiO_{4}. The double refraction, which is large in amount, is uniaxial in character, and positive in sign, the ordinary refractive index being 1·667 and the extraordinary 1·723. Its colour and softness distinguish it from peridot or diopside, which have about the same refractivity. The name was assigned to the species by Haüy, from διὰ, through, and ὄπτομαι, see, because the cleavage directions were distinguishable by looking through the stone. Dioptase has been found near Altyn-Tübe in the Kirghese Steppes, at Rezbánya in Hungary, and Copiapo in Chili, and at the mine Mindouli, near Comba, in the French Congo.