This species comes near beryl in chemical composition, being a silicate of aluminium and beryllium corresponding to the formula Be(AlOH)SiO_{4}, and closely resembles aquamarine in colour and appearance when cut. Owing to the rarity of the mineral good specimens command high prices for museum collections, and it is seldom worth while cutting it for jewellery. It derives its name from its easy cleavage, εὖ easily, and κλάσις fracture. The double refraction is biaxial in character and positive in sign, the least and greatest of the refractive indices being 1·651 and 1·670 respectively; the specific gravity is 3·07, and the hardness 7½ on Mohs’s scale. The colour is usually a sea-green, but sometimes blue. Euclase occurs with topaz at the rich mineral district of Minas Novas, Minas Geraes, Brazil, and has also been found in the Ural district, Russia. PHENAKITE Another beryllium mineral, phenakite owes its name to the frequency with which it has been mistaken for quartz, being derived from φέναξ, deceiver. The clear, colourless crystals, somewhat complex in form, have at times been cut, but they lack ‘fire,’ and despite their brilliant lustre meet with little demand. The composition is a silicate of beryllium corresponding to the formula Be_{2}SiO_{4}. The double refraction is uniaxial, and since the ordinary, 1·652, is less than the extraordinary index, 1·667, it is positive in sign; the specific gravity is 2·99, and the hardness is almost equal to that of topaz, being about 7½ to 8 on Mohs’s scale. Fine stones have long been known near Ekaterinburg in the Ural Mountains, and have recently been discovered in Brazil. BERYLLONITE As its name suggests, this mineral also contains beryllium, being a soda phosphate corresponding to the formula NaBePO_{4}. Clear, colourless stones, which occur at Stoneham, Maine, U.S.A., have been cut, but the lack of ‘fire,’ the easy cleavage, and comparative softness, the symbol being 5½ on Mohs’s scale, unfit it for use in jewellery. The double refraction is biaxial in character and negative in sign, the least and the greatest of the refractive indices being 1·553 and 1·565 respectively. CHAPTER XXXVI ENSTATITE, DIOPSIDE, KYANITE, ANDALUSITE, IDOCRASE, EPIDOTE, SPHENE, AXINITE, PREHNITE, APATITE, DIOPTASE ENSTATITE (‘_Green Garnet_’)