Phenacite, or phenakite, is the mineral form of beryllium silicate (Be2SiO4). It is an orthosilicate mineral found in pegmatities and high-temperature veins. Phenacite has hexagonal system rhombohedral crystals and is either colorless or glassy white, yellow, red or brown in color. It is sometimes used as a gem. Hardness 7.5–8; relative density 3.