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.