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Sphalerite (zinc sulfide)
Color: Green, brown, yellow, red, orange and colorless.
Moh’s hardness: 3.5-4
Refractive index: 2.268-2.371
Critical angle: 25°
Specific gravity: 4.08-4.1
Cleavage: Perfect in 6 directions.
Fracture: Uneven, brittle
Dispersion: Extremely strong (.156)
Heat sensitivity: Low
Birefringence (double refraction): None
Crystal structure: Cubic, isometric
Cutting: The multitude of cleavage planes are not as big of a problem as one would expect, but keep them away from the table. Use a fine lap to be safe.
Polishing: Alumina on tin/lead or wax lap.
Comments: The combination of a high RI and dispersion yields truly awesome cut stones.