Photo of Scheelite in Processed & Rough Form
Scheelite is a mineral with a hardness of 5 out of 10 on the Mohs scale of mineral hardness [?]. These Tetragonally structured gems are made of calcium tungstate, their full chemical compound being CaWO4.
Scheelite is a mineral in the group of Sulfates, Chromates, Molybdates, Tungstates. It has the same crystal structure and outside appearance as powellite. But scheelite is a calcium-tungstate, while powellite is a calcium-molybdate.
In rocks, scheelite may appear as crystals like two pyramids connected at its bases, or pseudo-octahedral, or having table-like faces, with detectable parallel ridges on its surface. It is rare to find scheelite in granular masses, where it is white or yellowish (especially when molybdenum-bearing).
It is semi-hard, very heavy, fragile, with good cleavage that enables it to be split along even planes. It is translucent to transparent, with vitreous to adamantine luster. It is fluorescent in short wave ultraviolet light, giving off a pale blue light which turns yellow in the molybdenum-rich member of the scheelite group. It is soluble in acids and fuses with difficulty. Its double refraction makes scheelite easy to distinguish from other similar-looking stones.
It is yellow, green, or reddish-gray. The yellow color resembles the colored diamond. In unpolished form it looks like crystallized caramel.
Mineral and crystal growths occur in high-temperature pegmatitic and hydrothermal veins, in Alpine fissures and medium-grade metamorphic rocks, including some metamorphic rocks.
Side by side with scheelite in high temperature veins and pegmatites is the related mineral wolframite, a grey-black to brownish-black monoclinic mineral of high specific gravity, which itself is the principal ore of tungsten outside the USA.
Splendid crystals weighing half a kilogram are found in Brazil, but the most important economic masses occur in Bolivia, Burma, Malaysia, Japan, China, the USA, and Austria. Fine crystals up to 7.5 cm. (3 in.) are found at Traversella (Turin), in Val di Fiemme and Valsugana (Trento) in Italy, and in various localities in Sarrabus and Gerrei in Sardinia. Fine pseudomorphs of ferberite-heubnerite after scheelite have been found at Trumbull, Connecticut (USA).
Tungsten is an obsolete term for scheelite.
Scheelite can be confused with quartz, with which it is commonly associated, but is distinguished by the characteristic electric blue to yellow fluorescence under ultraviolet light.
It is sometimes used as a diamond substitute.
What are the key gemological properties?
Crystal System: Tetragonal Refractive Index: 1.918-1.934 Birefringence: 0.016 Specific Gravity: 5.9-6.1 Hardness: 4.5-5 Mohs Perfect cleavage on {101}
What causes the distinctive fluorescence?
Fluorescence is typically brilliant blue-white under shortwave UV, caused by the tungsten content. This property made it historically important in tungsten prospecting. Intensity varies with composition and crystal purity.
What determines color variations?
Colors range from: Colorless to white: Pure material Yellow to brown: Iron impurities Green: Rare chromium content Grey: Mineral inclusions Color intensity varies with trace elements
What are key formation environments?
Forms in: Hydrothermal veins Skarn deposits Contact metamorphic zones Often associated with wolframite Common in tungsten-bearing deposits
What determines gem quality?
Transparency level Intensity of fluorescence Crystal form development Minimal inclusions Strong dispersion display Polish quality potential
What are the cutting considerations?
Challenges include: Moderate hardness limits durability Perfect cleavage requires careful orientation High dispersion affects cut choice Usually cut as collectors stones Rarely used in jewelry due to softness
What are key locality characteristics?
China: Large crystals common Austria: Historic locality South Korea: Fine fluorescent specimens USA: Notable in tungsten deposits
The specific gravity [?] for Scheelite is 6.1, its refractive index [?] is 1.92-1.93, and its double refraction [?] is 0.017.
History
Scheelite is named after Karl Wilhelm Scheele, the eighteenth century Swedish chemist who discovered 'tungsten,' which was the first name given to the stone by the Swedes. Scheelite was synthesized in the USA in 1963.
Industrial Usages
Scheelite is sometimes cut as very attractive faceted gems and prized by collectors. When dyed it can be confused with many gemstones. Cat's eye and four-rayed star scheelite are also known. It has also been manufactured synthetically.
Scheelite is an important ore of tungsten, especially in the USA. Tungsten is used to harden steels for use in high-speed tools and engine components, and to prepare tungsten carbides, abrasives which are second only to diamonds in hardness. Metallic tungsten has a high melting point - 3410°C (6170°F) - and is used in lamp filaments and spark plugs.
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