Density: 3.6 / Hardness: 7.5 to 8 / Varieties: Frequent octahedron, cube. Tetrahedron, dodecahedron rarer.
Transparent to translucent. All colours are possible: red, blue, green, brown; vitreous to dull lustre.
Spinels form in metamorphic contact rocks, gneisses, alluvial deposits.
They are often found in the same deposits as the ruby with which they have frequently been confused. On historical jewellery they are still called “broom rubies”.