Like diamond, spinel often crystallizes in perfect octahedral crystals. An octahedron is a three-dimensional shape that resembles two four-sided pyramids attached at their bases.

Octahedral crystals of spinel are often mined from marble, but they are more often found as water-worn crystals in stream sediments. Especially nice crystals are highly prized by mineral collectors or mounted as gems without being faceted. Natural crystals can make special gems.