Spinel is the magnesium/aluminum member of the larger spinel group of minerals.

Spinel is found as a metamorphic mineral in metamorphosed limestones and silica-poor mudstones. It also occurs as a primary mineral in rare mafic igneous rocks; in these igneous rocks, the magmas are relatively deficient in alkalis relative to aluminum, and aluminum oxide may form as the mineral corundum or may combine with magnesia to form spinel. This is why spinel and ruby are often found together.

High-quality blue or red spinel gemstones are actually very rare - even rarer than sapphire or ruby specimens of similar quality.