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 M2IIIMIIO4. Other members include chromite and magnetite. Synthetic spinels are used as refractories and in solid-state components.