Spinel

Related: Spinel Varieties

Black Spinel

Key Differentiators

• Singly refractive
• Characteristic octahedral crystals
• Strong fluorescence in red/pink varieties

Natural vs. Synthetic

Synthetic spinel is commercially available (Flame fusion (Verneuil — non-stoichiometric MgAl2O4), Flux (stoichiometric composition, rare)). Distinguishing natural from synthetic typically requires microscopic examination of internal features.

• RI: Natural spinel RI ~1.712–1.730 (most gem quality 1.715–1.725, varies with Fe content). Synthetic: Flame fusion RI ~1.727 — distinctly higher than most natural spinel due to excess alumina. A reading at or above 1.727 is a strong red flag.
• Polariscope (ADR): Weak to moderate ADR — faint wavy or patchy extinction, not patterned. Synthetic: Flame fusion: strong tabby extinction — checkerboard ADR pattern under crossed polarizers. Diagnostically useful. Flux-grown: minimal ADR (close to natural).
• Microscopy: Natural inclusions: angular/hexagonal negative crystals, irregular fluid inclusions, growth zoning. Octahedral inclusions, iron-ore crystals. Synthetic: Flame fusion: curved growth striae (concentric) and spherical or tadpole gas bubbles in rows. Flux-grown: wispy flux veils, possible platinum platelets; no striae.
• Chelsea Filter (blue spinel): Natural blue spinel: green to brownish-green under Chelsea filter. NEVER red. Synthetic: Cobalt-doped blue flame fusion: strong red to orange-red under Chelsea filter. Diagnostically useful — immediate separation from natural blue spinel.
• UV Fluorescence: Natural red/pink spinel: moderate to strong red-orange LW UV, transparent quality. Natural blue spinel: typically inert (iron quenches). Synthetic: Flame fusion: strong, chalky/opaque-looking fluorescence. Red/pink: chalky orange-red. 'Chalky' quality of fluorescence (dull, opaque, not transparent) is characteristic.

Common Simulants

• Ruby: Ruby: doubly refractive uniaxial negative — dichroism visible under dichroscope; Chelsea filter strongly red; RI 1.762–1.778.
• Sapphire: Sapphire: doubly refractive uniaxial negative; dichroism; RI 1.762–1.778; SG 4.00; different inclusions.
• Red garnet: Both isotropic; distinguish by RI (garnet 1.720–1.790 vs spinel ~1.718) and absorption spectrum.

Commonly Confused With

Commonly confused with: Ruby, Sapphire, garnet, Almandine Garnet, Pyrope Garnet, Rhodolite Garnet, Rubellite, Star Ruby, Star Sapphire, Zircon.

Treatments

• Heat Treatment (uncommon — may lighten dark stones)
• Fracture Filling (glass/resin)
• Cobalt Diffusion Treatment (surface coating/diffusion — detectable by LA-ICP-MS)
• Nickel Diffusion Treatment (blue-green color from Ni; detected by photoluminescence + LA-ICP-MS)

Price Context

Price context is approximate. GemID is not an appraisal tool. Results are indicators, not certified valuations.

About Spinel

A durable, singly refractive gemstone that occurs in a wide range of colors.