Phenacite or phenakite is a nesosilicate mineral consisting of beryllium orthosilicate, Be₂SiO₄. Occasionally used as a gemstone, Phenacite 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. There is no cleavage, and the fracture is conchoidal. The hardness is high, being 7.5 - 8; the specific gravity is 2.96. The crystals are sometimes perfectly colorless and transparent, but more often they are greyish or yellowish and only translucent; occasionally they are pale rose-red. In general appearance the mineral is not unlike quartz, for which indeed it has been mistaken.

Phenacite is found in high-temperature pegmatite veins and in mica-schists associated with quartz, chrysoberyl, apatite and topaz. It has long been known from the emerald and chrysoberyl mine on the Takovaya stream, near Ekaterinburg in the Urals of Russia, 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 Pikes Peak region in Colorado (USA). Large crystals of prismatic habit have more recently been found in a feldspar quarry at Kragero in Norway. Framont near Schirmeck in Alsace is another well-known locality. Still larger crystals, measuring 1 to 2 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 are higher than those of quartz, beryl or topaz; a faceted phenacite is consequently rather brilliant and may sometimes be mistaken for diamond.