Spinel

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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

RI Range1.712–1.736

SG Range3.57–3.63

SG Typical3.60

Hardness (Mohs)7.5–8

Crystal SystemCubic

Optic CharacterSR (Singly Refractive)

Dispersion0.020

Fluorescence LWVariable

Fluorescence SWInert

Chelsea FilterVariable

PleochroismNone

ColorsRed Pink, Blue Violet, Purple, Black, Colorless, Green, Yellow Orange

SpeciesSpinel

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.

GemID Pro includes a two-phase natural vs. synthetic testing protocol for Spinel.

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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

Natural — low ($/ct)$200

Natural — high ($/ct)$3,000

NotePer carat; fine red Burmese and cobalt blue command premiums; undervalued historically

Synthetic — low ($/ct)$5

Synthetic — high ($/ct)$30

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

Related Comparisons

Measurement Guides

Identifying a spinel? GemID walks through these tests in order — RI, SG, fluorescence, and more.

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Studying for the GIA, FGA, or FEEG practical? Spinel is one of the 130 species in GemID’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.

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