Spinel is an oxide mineral, magnesium aluminum oxide (MgAl₂O₄), found in igneous and metamorphic rocks which formed at high temperatures. It crystallizes in the cubic system, frequently forming twinned, octahedral crystals. Spinel has a glassy appearance and is black, red, blue, brown, or white in color. Ruby spinel from Sri Lanka is a valuable gemstone. Hardness 7.5–8, relative density 3.8.

Spinel itself is the chief member of the spinels group of minerals of general formula M₂^IIIM^IIO₄. Other members include chromite and magnetite. Synthetic spinels are used as refractories and in solid-state components.