Spinel

Spinel (MgAl₂O₄) is a crystalline oxide belonging to the *spinel* group, a family of minerals with cubic structure characterized by magnesium ions in octahedral coordination and aluminum ions in tetrahedral coordination. Its hardness of 8 on the Mohs scale makes it ideal for jewelry, inferior only to diamond (10) and corundum (9). The most prized red variety, *red spinel* or *balas ruby*, was long confused with ruby until modern crystallography clarified the chemical and structural differences. Color depends on trace elements such as chromium (red), iron and cobalt (blue), and vanadium (green). The most important deposits are found in Burma (Myanmar), Sri Lanka, Tanzania, and Madagascar, with Burmese crystals historically considered the finest. The contemporary gemological market particularly appreciates saturated red and blue spinels, which command high prices when pure and well-cut.

Crystal system: *cubic*, space group Fd-3m, lattice parameter a = 8.08 Å. Density: 3.58–3.61 g/cm³. Refractive index: nω = 1.712–1.735 (isotropic, uniaxial negative in stressed varieties). Birefringence: absent (pure crystals); may manifest in samples with compositional zonation. Chromatic dispersion (fire): 0.020. Hardness: 8 Mohs; conchoidal fracture. Cleavage: none (fracture). UV-Vis spectroscopy: characteristic absorption in the blue (Fe²⁺–Fe³⁺ charge transfer) and in the red (d-d transitions of Cr³⁺). Raman spectroscopy: main band at ~680 cm⁻¹ (Mg–O vibration). Fluorescence: variable; red spinels often inert under UV, blue spinels may show weak red fluorescence. Composition: pure MgAl₂O₄; common ionic substitutions include Fe²⁺, Zn²⁺ (replacing Mg), Fe³⁺, Cr³⁺, V³⁺ (replacing Al). Formation: typically in high-grade metamorphic rocks (granulites, eclogites), dolomitic marbles, granitic pegmatites, and in association with corundum, feldspars, olivine. Treatments: occasionally heating to improve color; rarely oiling. Distinction from ruby: lower density (3.6 vs 3.99 g/cm³), lower refractive index (1.71–1.73 vs 1.76–1.77), absence of birefringence, distinct absorption spectrum.