PHENACITE  , a

mineral consisting of beryllium orthosilicate, Be2SiO4, occasionally used as a gem-stone . It occurs as isolated crystals, which are rhombohedral with parallel-faced hemihedrism, and are either lenticular or prismatic in habit: the lenticular habit is determined by the development of faces of several obtuse rhombohedra and the absence of prism faces (the accompanying figure is a plan of such a crystal viewed along the triad, or principal, axis) . There is no cleavage, and the fracture is conchoidal . The hardness is high, being 71—8; the specific gravity is 2.98 . The crystals are sometimes perfectly colourless and transparent, but more often they are greyish or yellowish and only translucent; occasion-ally they are pale rose-red . In general appearance the mineral is not unlike quartz, for which indeed it had been mistaken; on this account it was named, by N . Nordenskiold in 1833, from Gr . OivaE (a deceiver) . Phenacite has long been known from the emerald and chrysoberyl mine on the Takovaya stream, near Ekaterinburg in the Urals, where large crystals occur in mica-schist . It is also found with topaz and amazon-stone in the granite of the Ilmen mountains in the southern Urals and of the Pike's Peak region in Colorado . Large crystals of prismatic habit have more recently been found in a felspar quarry at Kragero in Norway . Framont near Schirmeck in Alsace is another well-known locality .

Still larger crystals, measuring 12 in. in

diameter and weighing 28 lb, have been found at Greenwood in Maine, but these are pseudomorphs of quartz after phenacite . For gem purposes the stone is cut in the brilliant form, of which there are two fine examples, weighing 43 and 34 carats, in the British Museum . The indices of refraction (w=1.6540, e=1.6527) are higher than those of quartz, beryl or topaz; a faceted phenacite is consequently rather brilliant and may sometimes be mistaken for diamond . (L . J .