Spinel, magnesium aluminium oxide,
Crystal system: Cubic (crystals usually octahedra
Density: 3.6-4.1 gms/cm3
Colour: Red when pure but can be colourless, yellow, green, or blue.
Magnesium-aluminium Spinel is a member of the much larger Spinel Group (that includes Rubies) having the general formula (Mg,Fe)(Al,Cr)2O4. It occurs in magmatic and contact metamorphic rocks. Occurrence is rare. Primary use is as gemstones.
The picture shows lovely blue terminated spinel crystals from Aliabad, Hunza Valley, Pakistan.