The hardness of sphalerite is relatively low, 3.5-4 in the Mohs scale, which is the reason to use these gems mainly for collections. Moreover, sphalerite presents a perfect cleavage in 6 different directions, corresponding to faces of a rhombic dodecahedron, which means additional faceting difficulties.
The specific gravity of sphalerite is 3.9-4.2, practically the same value as ruby and sapphire. The streak is light brown to yellowish white. The colors of Spainsh sphalerites are considered in this special section. Optic absorption spectra of orange stones usually present three lines at 690, 667 y 651 nm.
The refractive index of sphalerite is 2.369, almost as high as diamond's 2.42. That's why sphalerite has very strong adamantine lustre on polished and exfoliated surfaces. The lustre in natural sphalerite samples from Áliva is very variable; it can vary between adamantine in the exfoliated planes and resinous or waxy in natural crystal faces. Even in the same sample there can be some very brilliant and some matt faces.
One of the most relevant optical properties of sphalerite is its high dispersion of light, almost four time higher to the diamond’s one (0,156 against 0.044). In gemology, dispersion is defined as the difference in the refractive index of a mineral at the blue and red light (686.7 nm and 430.8 nm respectively). The dispersion is responsible of the splendid "fire" effect observed in faceted sphalerites, especially in lightly colored yellow and green stones. Strong punctual light source is the best lighting condition for dispersion observation.
Ray-trace modeling for faceted sphalerite showing the dispersion of light for refracted rays.
For more information on ray-trace study of spahlerites before faceting please see corresponding section.
Flashes of all spectral colors observed in faceted yellow sphalerite due to its high dispersion.
"Markoh-i-noor" cut, 31.48 ct stone, diameter 18.2 mm.
Video of the same stone with outstanding dispersion.
Raman spectrum of sphalerite from RRUFF database