Spinel
Last updated: April 2026
A durable, singly refractive gemstone that occurs in a wide range of colors.
Spinel has a specific gravity of 3.60 and a refractive index of 1.712–1.736, with a Mohs hardness of 7.5–8.0. It is singly refractive.
Physical & Optical Properties
Measured with a polariscope, a refractometer, a hydrostatic balance, a Chelsea filter, a UV torch, a loupe, a darkfield loupe and a spectroscope — step-by-step for each.
Through the refractometer
Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.
Related: Spinel Varieties
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.
Loupewise Pro includes a two-phase natural vs. synthetic testing protocol for Spinel.
Common Simulants
- Ruby: Ruby: doubly refractive uniaxial negative — dichroism visible under dichroscope; Chelsea filter strongly red; RI 1.762–1.770.
- Sapphire: Sapphire: doubly refractive uniaxial negative; dichroism; RI 1.762–1.770; 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)
Related Comparisons
Measurement Guides
Identifying a spinel? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →Studying for the GIA, FGA, or FEEG practical? Spinel is one of the 132 species in Loupewise’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.
Practice exams →