Spinel, magnesium aluminium oxide,
Formula MgAl2O4

Crystal system: Cubic (crystals usually octahedra or dodecahedra)
Hardness: 7.5-8
Density: 3.6-4.1 gms/cm3
Colour: Red when pure but can be colourless, yellow, green, or blue.
Streak: White.

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.

Blue Spinel from Aliabad, Hunza Valley, Pakistan.