Spinel
Spinel
Spinel, like garnet and diamond, is singly refractive, with the same physical properties in all crystal directions. It belongs to the cubic crystal system, and its characteristic crystal shape is an octahedron, which looks like two back-to-back pyramids. Well-formed spinel crystals are fairly common in nature. Spinel can also form flattened crystals that look radically different from octahedral crystals. The flattened shape occurs when the pyramids that form an octahedron rotate against each other during growth.
