Scheelite
Scheelite is a calcium tungstate mineral according to the chemical formula CaWO4. It is an important ore of tungsten. Well-formed crystals are sought by collectors and are occasionally fastened into gemstones when suitably free of flaws. Scheelite has been synthesized via the Czochralski process; the material produced may be used to imitate diamond, as a scintillator, or as a solid state lasing medium.
Its crystals are in the tetragonal crystal system, appearing as dipyramidal pseudo-octahedra. Colours include golden yellow, brownish green to dark brown, pinkish to reddish gray, and colourless. Transparency ranges from translucent to transparent and crystal faces are highly lustrous (vitreous to adamantine). Scheelite possesses distinct cleavage and its fracture may be subconchoidal to uneven. Its specific gravity is high at 5.9–6.1 and its hardness is low at 4.5-5. Aside from pseudo-octahedra, scheelite may be columnar, granular, tabular or massive in habit. Twinning is also commonly observed and crystal faces may be striated. Scheelite refractive index (1.918-1.937 uniaxial positive, with a maximum birefringence of 0.016) and dispersion (0.026) are both moderately high. These factors combine to result in scheelite's high lustre and perceptible "fire", approaching that of diamond. Owing to low hardness, however, cut scheelites are best enjoyed unset as valuable collector's pieces.
Rockhounds treasure scheelite for its fluorescent properties: under shortwave ultraviolet light, the mineral glows a bright sky-blue.
Scheelite is associated with wolframite in placer deposits and forms in pegmatites, hydrothermal veins, and areas of contact metamorphism. Other associated minerals include fluorite, muscovite, dolomite and molybdenite. Its type locality is the Bisperg iron mine in Sater, Dalarna County, Sweden. Officially recognized in 1821, scheelite was named after Swedish chemist Karl Wilhelm Scheele, the discoverer of tungsten.
Crystals exceeding 0.5 kilograms (1 pound) have been found in Brazil: Other notable localities include Australia, Austria, Bolivia, Burma, England, Finland, France, Italy, Japan, Sri Lanka, Switzerland and the United States. The Sichuan Province of China has emerged as a newly important source with many "gemmy" specimens recovered. Fine crystals are also found in Tong Wha, Korea.
