Physical properties
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. For optic absorption spectra and other spectroscopic characteristics of sphalerite, please refer to this section.
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.
Comparison of the dispersion of some common gems (click to enlarge)
Data sources: ClassicGems.net, The Gemology Project
Flashes of different spectral colors observed in faceted yellow sphalerite due to its very high dispersion.
20.30 ct, elongated hexagon barion cut. Design, faceting and video by Egor Gavrilenko.
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.
Other example of faceted yellow sphalerite with outstanding dispersion.
"Markoh-i-noor" cut, 31.48 ct stone, diameter 18.2 mm.