Species and chemistry
Spinel is magnesium aluminum oxide (MgAl₂O₄). Mohs hardness is 8; specific gravity is 3.58 to 3.61. Spinel is isotropic — singly refractive — with no pleochroism.
Color in spinel is produced by trace-element substitution. Chromium produces red; cobalt produces a saturated blue; iron and zinc produce a range of pinks, purples, and gray-blues.
Sources
- Vietnam — Luc Yen district: source of saturated red and pink spinel.
- Sri Lanka — Ratnapura and Elahera: historic and continuing source of a wide spinel color range.
- Tanzania — Mahenge: source of the high-saturation pink-red "jedi" spinel found since the early 2000s.
- Myanmar — Mogok valley: historic source of red spinel of premium quality.
- Tajikistan — Pamir Mountains: source of pink and red spinel.
Historical context
Spinel was historically conflated with ruby in royal collections; the "Black Prince's Ruby" in the British Crown Jewels and the "Timur Ruby" are both spinels. Distinguishing spinel from ruby was not reliably possible until 19th-century gemmological techniques became available. The species has historically been undervalued relative to its optical properties and hardness.
Treatments
Spinel is largely untreated. Heat treatment is occasionally applied to improve clarity in cobalt-bearing material; the practice is uncommon and is generally disclosed. Synthetic spinel (Verneuil) has been commercially available since the early 20th century and is identifiable in the laboratory.
Lapidary considerations
Spinel cuts cleanly with diamond grits; the species is hard and isotropic, simplifying preform orientation. Final polish is conventionally performed at 50,000 mesh diamond paste or with a tin-lead master plate. Octahedral crystal habit makes spinel rough easy to evaluate for clarity prior to preforming.