Scheelite

Scheelite is calcium tungstate, a major ore of tungsten alongside wolframite and a favorite of fluorescent-mineral collectors. It crystallizes in the tetragonal system, forming characteristic bipyramidal (dipyramidal) crystals that look like double pyramids, as well as granular and massive aggregates.

Colors include white, gray, yellow, orange-brown, and colorless, with a vitreous to adamantine luster and a high refractive index that gives clear crystals notable brilliance. The mineral is named for the chemist Carl Wilhelm Scheele, who first identified tungsten from it.

Scheelite's signature feature is a brilliant blue-white to yellow fluorescence under short-wave ultraviolet light, long used by prospectors to find tungsten ore in the dark.

Scheelite forms mainly in contact-metamorphic skarn deposits, where hot, tungsten-bearing fluids from a granitic intrusion react with carbonate host rocks such as limestone, depositing scheelite with garnet, diopside, epidote, and other calc-silicates.

It also occurs in high-temperature hydrothermal quartz veins and greisens, often with wolframite, cassiterite, molybdenite, and gold, and in some pegmatites. Because it is fairly dense, scheelite can concentrate in placers. Notable localities include the skarns of China and South Korea, Mittersill in Austria, Sardinia and Traversella in Italy, Tae Wha in Korea, and many tungsten districts in the western United States.

The single best test for scheelite is ultraviolet fluorescence: it glows bright blue-white (shifting toward yellow with higher molybdenum content) under short-wave UV, which quickly separates it from most look-alikes. In daylight, look for white to yellow-brown bipyramidal crystals with a vitreous to adamantine luster, a white streak, and hardness 4.5-5.

Scheelite is fairly heavy. It can resemble quartz, calcite, or feldspar in hand specimen, but those lack its weight and strong blue UV response (calcite fluoresces differently and fizzes in acid). The double-pyramid crystal form plus blue fluorescence is essentially diagnostic.

Scheelite is an important ore of tungsten, supplying the metal for tungsten carbide cutting tools, drill bits, wear-resistant components, high-temperature alloys, lamp filaments, and heavy alloys. Many large modern tungsten mines work scheelite-bearing skarns.

Synthetic calcium tungstate (and related tungstates) has been used as a scintillator and phosphor, exploiting the same luminescence seen in natural scheelite, and historically in X-ray intensifying screens. Transparent crystals are occasionally faceted as brilliant but soft collector gems. Bright fluorescent specimens are popular with collectors, and the UV glow remains a practical prospecting aid.