Some gemstones display a rare property known as fire. When you turn a gemstone in the light, some gems will display flashes of color, which is the result of the dispersion of white light into the colors of the spectrum. Dispersion occurs when different wavelengths of light are refracted by a different amount by the refracting medium. Diamond is famed for this quality, but a number of other gemstones are notable for their fire, including demantoid garnet, sphene and zircon.
However, there is a little-known gem which has even greater fire; in fact its dispersion rating is three times as high as that for diamond. The gem is called sphalerite and gem dealers who specialize in collector's gems will sometimes have specimens in stock.
Sphalerite is a zinc sulphide and is best known in the mineral world as the chief ore of zinc. Most sphalerite contains iron and when the iron content is high it is an opaque black. The rare gem-quality crystals have a very low iron content and considerable transparency. They are typically a yellowish to honey brown, red to orange, or green.
Sphalerite is a relatively soft gem, with a hardness of only 3.5 to 4 on the Mohs scale, about the same as fluorite. So despite its amazing fire, it is not suitable for rings. It has a density or specific gravity of 3.90-4.10, making it one of the denser gems, similar to sapphire and ruby, but not quite as dense as zircon. In addition to its outstanding dispersion, it has a very high refractive index of 2.368-2.371, just slightly lower than diamond and substantially higher than zircon, spessartite garnet and sapphire. Sphalerite can have outstanding luster and it is one of very few gems that are classified as having adamantine or diamond-like luster. Like diamond, sphalerite forms as a cubic crystal and has perfect cleavage. Clean specimens are very rare except in small sizes.
The two most important sources for gem-quality sphalerite are the Chivera mine in Sonora, Mexico; and the Picos de Europa, near Santander on Spain's northern coast. Sphalerite is also found in Namibia and the Congo.
Collectors search out sphalerite specimens for its remarkable dispersion. But it is important to point out that gems with a darker body color will exhibit less fire than lighter colored gems. In general we can say that large light colored stones with very good clarity will display the greatest dispersion.
First Published: December-09-2008
- First Published: December-09-2008
- Last Updated: February-26-2011
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