Sphalerite

(Blende or Zinc Blende)
Chemistry: Zinc sulphide
Crystal system: Cubic; generally in tetrahedral but also cube, octahedral, dodecahedral and trisoctahedral; twins common; massive.
Color: Transparent to translucent; yellow, green, colorless; non-gem nearly black in color.
Hardness: 3.5 - 4
Cleavage: Perfect: dodecahedral (6 directions); fracture: brittle, uneven.
Specific gravity: 4.09
Refractive index: 2.37; SR
Luster: Resinous to adamantine
Dispersion: Very high
Dichroism: -
Occurrence: Chief ore of zinc; Central Africa; Mexico; Spain.

Notes
Collector’s stone; difficult to cut due to cleavage; soft; scratches easily; has been used as prism in high R.I refractometers; may look like zircon; spectrum: 3 bands in red-690,667,651nm; faceted.