Spinels

Spinel refers to a group of minerals that share the cubic spinel structure and a general AB2O4 formula. The end-member MgAl2O4 (magnesium aluminum oxide) is the characteristic spinel mineral, and many related minerals fit within the spinel group, including chromite (FeCr2O4) and magnetite (Fe3O4). In the crystal structure, oxide ions form a close-packed lattice; the A cations occupy tetrahedral sites and the B cations occupy octahedral sites. Some spinels show inversion, with partial exchange of A and B cations.

Spinels crystallize in the isometric system, typically forming octahedral or near-octahedral crystals. They have a hardness of 8 on the Mohs scale, a specific gravity of about 3.5–4.1, and a vitreous luster. They are usually transparent to translucent and can be colorless or colored by transition metals; red spinel is commonly chromium-doped MgAl2O4, while blue and green varieties reflect other impurities or color centers. Spinel is usually not cleaved and fractures conchoidally.

Spinel minerals occur in both metamorphic and igneous rocks. Chromium-rich spinel occurs in ultramafic rocks such as peridotite and in high-grade metamorphic belts, while gem-quality crystals form in marble-hosted environments and in alkali-rich pegmatites and basalts. Major gem sources include Sri Lanka, Myanmar, Afghanistan, Tanzania, Vietnam, and several others.

Spinel has long been used as a gemstone and is valued for its range of colors and durability. Historically some red spinel has been mistaken for ruby; the Black Prince's Ruby in the British Imperial State Crown is a famous spinel historically regarded as a ruby. Color in spinel can be enhanced by heat treatment; blue spinel may be produced by cobalt-related diffusion, and synthetic spinel has been used as a gemstone substitute since the early 20th century.