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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

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
Red PinkBlue VioletPurpleBlackColorlessGreenYellow Orange

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

Spinel varies 1.712 – 1.736 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 REFRACTIVE INDEX DORotate the stone through a full 360°, watching the boundary. SEEOne boundary that does not move as you rotate. Where it sitsis not fixed for this species, though: published at 1.712 –1.736, and the stone drawn here reads 1.724, between thedashed marks. MEANSSingly refractive — a stationary single edge through thewhole rotation says so. The value itself is only anarrowing: this species spans 2 printed divisions, so areading places your stone inside the range rather thannaming it. Computed from gems.json — edge positions are arithmetic, not drawn. RI increases upward; the field above the boundary is the dark one.

Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.

Through the polariscope

Spinel Dark throughout (SR) 0° 45° 90° 135° ROTATION BETWEEN CROSSED POLARS Spinel stays dark through the whole rotation. That is singly refractive: there is no second ray to rotate the plane of vibration back into the analyser. Computed from optic character and transparency in gems.json. A polariscope separates singly refractive, doubly refractive and aggregate — it does not show the optic sign.

Drawn from the optic character and transparency above. A polariscope separates singly refractive, doubly refractive and aggregate; the optic sign needs a conoscope. How to take this reading.

Specific gravity, and what it rules out

Spinel SG 3.57–3.63 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 12 other species in the table overlap this range, so a reading here narrows the field to 13 rather than naming the stone. Combine it with a refractive index to separate what is left. Computed from the specific-gravity range in gems.json. Hydrostatic weighing — the bench method this site recommends; heavy liquids are toxic and regulated. Faint ticks are the other species in the table.

Drawn from the specific-gravity range above, against every other species in the table. How to take this reading.

Through the dichroscope

Spinel One colour (no pleochroism) IDENTICAL AT EVERY ANGLE Spinel shows no pleochroism, so both windows match at every orientation — turning the instrument changes nothing. That is a real negative result, and the reason it is worth knowing is below. Computed from pleochroism and pleochroic colours in gems.json. Window tints are derived mechanically from the colour words, not chosen; a neutral pair depicts no colour at all.

Drawn from the pleochroism above. Two windows that match mean either no pleochroism or the wrong angle — turn the instrument to tell. How the dichroscope compares.

Black Spinel

Key Differentiators

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.

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

Common Simulants

Commonly Confused With

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

Treatments

Related Comparisons

Natural Vs Synthetic SpinelPyrope Garnet Vs SpinelRuby Vs Red Spinel

Measurement Guides

How to Identify a GemstoneHow to Measure Specific GravityHow to Use a RefractometerUV Fluorescence Testing

Identifying a spinel? Loupewise 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 132 species in Loupewise’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.

Practice exams →