spinell

Spinell, also known as spinel, is a mineral species and a widely used gemstone. Its chemical formula is MgAl2O4, and it belongs to the spinel group with the general AB2O4 composition. The crystal structure is cubic, with divalent A2+ ions occupying tetrahedral sites and trivalent B3+ ions occupying octahedral sites in a close-packed oxide lattice. Natural spinell typically forms octahedral crystals and can occur in granular aggregates.

Spinell has a Mohs hardness of 8, a vitreous luster, and a specific gravity around 3.58–3.60. The refractive index is about 1.718, and because spinell is cubic it is isotropic with no pleochroism. It occurs in a broad color range including colorless, red, pink, orange, yellow, green, blue, brown, and black. Red to pink colors usually arise from chromium; blue spinell can be cobalt-doped or contain other impurities.

Spinell forms in metamorphic rocks and ultramafic igneous rocks, often associated with chromite, olivine, and pyroxene. It is found in many localities worldwide, including Myanmar (Burma), Sri Lanka, Afghanistan, Tanzania, Vietnam, and Madagascar. Historically, red spinels from various sources were called balas rubies, and several famous stones, such as the Black Prince's Ruby, are spinels rather than corundum gems.

Spinell has long been valued as a gemstone for its brilliance and durability and is often priced more accessibly than ruby. Synthetic spinell has been produced since the early 20th century by processes such as Verneuil and flux growth, and is used in jewelry and as a laboratory gemstone. Natural spinell colors can be enhanced by heat treatment, and some stones may be irradiated or fracture-filled, though such treatments are less common for high-quality gem spinell.