EuclaseTHIS species comes near beryl in chemical composition, being a silicate of aluminium and beryllium corresponding to the formula Be(AlOH)SiO4, 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, e? easily, and ???s?? 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. PhenakiteAnother beryllium mineral, phenakite owes its name to the frequency with which it has been mistaken for quartz, being derived from f??a?, Fine stones have long been known near Ekaterinburg in the Ural Mountains, and have recently been discovered in Brazil. BerylloniteAs its name suggests, this mineral also contains beryllium, being a soda phosphate corresponding to the formula NaBePO4. 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. |