Scheelite

Crystal system Tetragonal
Transparency Transparent to translucent
Luster Sub-Adamantine
Fracture Uneven to Conchoidal
Cleavage distinct prismatic
Specific Gravity 6.12
Hardness 5
Optical Character Uniaxial+ Double Refractive
Refractive index 1.918 - 1.934
Birefringence 0.016
Dispersion moderate
Fluorescence strong light blue under s.w.; inert under l.w.
Pleochroism orange stones - weakly dichroic
Chemical Formula CaWO4
Comments
Streak White

Scheelite, a calcium tungstate with the formula CaWO4, is the most important ore of tungsten. It is named after Karl Wilhelm Scheele (1742-1786), the Swedish chemist.

This mineral occasionally occurs in colorless, orange, yellow or brown transparent crystals, which may be cut for collectors, but most crystals are etched and smooth faces are rare. The transparent crystals, when faceted, show considerable dispersion.

Scheelite is produced synthetically in any color; it may be separated from natural by its inclusions, spectra and fluorescence. A spectrum has not been recognized in the orange material.

In the United States, sources of scheelite are Connecticut, Utah, Arizona, and California. It is also mined in England, Germany, Czechoslovakia, and the Italian Piedmont, the Swiss Alps, Spain, Peru, Japan, and Korea. Mexico, Finland, and Australia are also sources.

Few of the above sources produce material transparent enough to be used as gemstones. However, in recent years fine yellow to orange specimens have been found in Sonora, Mexico, which have produced cut stones over ten carats. Good, cuttable material is also found in Arizona and California. The transparent, vivid orange-brown color is especially desirable and large crystals bring large prices. Fine yellow stones may resemble colored diamond, but they can be separated easily by the strong double refraction of scheelite.

Scheelite is quite brittle, although cleavages are not troublesome. It is not sensitive to heat and is practically infusible. It should be pre-polished on tin with Linde A and finished on a wax lap again with Linde A. It rarely polishes perfectly, however, and facet edges tend to chip. For faceted stones, crown angles of 35° and pavilion angles of 41° are recommended.