Sphalerite

Crystal system Cubic
Transparency Transparent to opaque
Luster Adamantine to resinous
Fracture Conchoidal
Cleavage Perfect dodecahedral
Specific Gravity 4.05
Hardness 3.5 to 4
Optical Character Single Refractive
Refractive index 2.37
Birefringence
Dispersion 0.156
Fluorescence
Pleochroism generally inert; may fluoresce orange
Chemical Formula ZnS
Comments Attacked by acids
Streak White to pale yellow or brown

Although sphalerite (SFAL-er-ite) has a hardness of only 3 1/2 to 4 and possesses six directions of perfect dodecahedral cleavage, transparent stones are highly prized by collectors because of the mineral's tremendous dispersion (0.156) and R.I. (about 2.37). The resultant brilliancy and fire displayed by green, greenish-brown, yellow-brown or reddish-brown stones make a very pleasing appearance.

The word sphalerite is derived from the Greek word meaning "treacherous", and the alternate name, blende, is from the German "deceive", because it was often mistaken for galena, the lead-sulphide mineral with which it is often associated.

Sphalerite, the principal ore of zinc, bears the simple chemical formula ZnS (zinc sulphide). It crystallizes in the cubic system
(and is therefore singly refractive), often in well-formed tetrahedrons and occasionally in tristetrahedrons, cubes and dodecahedrons, usually distorted and rounded or otherwise modified; twins are common. The mineral is found most commonly, however, in compact, coarse to fine granular cleavable masses. Other properties, in addition to those mentioned above, are as follows : luster, adamantine to resinous; S.G. 3.90 to 4.20, but gem quality is usually near 4.05; toughness, very poor; streak, white to pale yellow to pale brown; fracture, conchoidal. It fuses with difficulty but is attacked by hydrochloric acid. Fluorescence and triboluminescence and sometimes observed.

This commercially important mineral occurs both in veins and in irregular deposits in limestone, often associated with galena, pyrite, chalcopyrite and other sulphides. Important sources of gem- quality sphalerite include Cannnea, Sonora, Mexico and Picos de Europa near Santander, Spain.

Because of the softness and perfect cleavage of sphalerite, it is obvious that fashioning is very difficulty and hazardous. Linde A powder on a tin lap, using crown and pavilion angles of 35° and 41° respectively, produces a good polish and utilizes the mineral's high R.I. and dispersion to best advantage. If pitting appears, changing polishing directions will usually overcome the difficulty. A finish polish, using Linde A on a wax lap, will often increase the luster.