sphalerite

A zinc sulphide (ZnS) mineral, also known as zinc blende. Colourless when pure, it often contains iron, which darkens its colour. It occurs in hydrothermal veins, usually associated with galena and silver minerals. It is the principal ore of zinc.

Sphalerite (ZnS) is a mineral that is the chief ore of zinc.

The mineral crystallizes in the cubic crystal system. In the crystal structure, zinc and sulfur atoms are tetrahedrally coordinated. The structure is closely related to the structure of diamond. The lattice constant for zinc sulfide in the zincblende crystal structure is 0.542 nm.

Its color is usually yellow, brown, or gray to gray-black, and it may be shiny or dull. Some specimens have a red iridescence within the gray-black crystals; The refractive index of sphalerite (as measured via sodium light, 589.3 nm) is 2.37. Sphalerite crystallizes in the isometric crystal system and possesses perfect dodecahedral cleavage. Gemmy, pale specimens from Franklin, New Jersey (see Franklin Furnace) are highly fluorescent orange and/or blue under longwave ultraviolet light and are known as cleiophane, an almost pure ZnS variety.

Crystals of suitable size and transparency have been fashioned into gemstones, usually featuring the brilliant cut to best display sphalerite's high dispersion of 0.156 (B-G interval)—over three times that of diamond. Freshly cut gems are lively with an adamantine luster and could conceivably be mistaken for a fancy-colored diamond in passing, but due to sphalerite's softness and fragility the gems are best left unset as collector's or museum pieces (although some have been set into pendants). Collectors may pay a premium for stones over one carat (200 mg), as clean crystals are usually quite small. Gem-quality material is usually a yellowish to honey brown, red to orange, or green;