Spinel

Spinel is the magnesium/aluminium member of the larger spinel group of minerals. Its name comes from the Latin word spinella, a diminutive form of spine, in reference to its pointed crystals.

Spinel crystallizes in the isometric system; common crystal forms are octahedra, usually twinned. Its hardness is 8, its specific gravity is 3.5–4.1, and it is transparent to opaque with a vitreous to dull luster. It may be colorless, but is usually various shades of red, lavender, blue, green, brown, black, or yellow.

This stone found as a metamorphic mineral in metamorphosed limestones and silica-poor mudstones. It also occurs as a primary mineral in rare mafic igneous rocks.