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Synthetic Spinel

Last updated: August 2026

Flame-fusion synthetic spinel was the dominant birthstone simulant from the 1930s–1980s. Estate jewelry from this era frequently contains it as a simulant for sapphire, aquamarine, ruby, and alexandrite. The cobalt-blue variety shows a strong red Chelsea filter reaction. All varieties show anomalous double refraction ('tabby extinction') on the polariscope. RI is slightly elevated vs. natural spinel due to excess alumina in the Verneuil process.

Synthetic Spinel has a specific gravity of 3.63 and a refractive index of 1.725–1.728, with a Mohs hardness of 8. It is singly refractive.

Physical & Optical Properties

RI Range1.725–1.728
SG Range3.61–3.66
SG Typical3.63
Hardness (Mohs)8
Crystal SystemCubic
Optic CharacterSR (Singly Refractive)
Dispersion0.020
Fluorescence LWStrong Blue White
Fluorescence SWStrong Blue White
Chelsea FilterRed
PleochroismNone
ColorsBlue Violet, Red Pink, Colorless, Green, Yellow Orange
SpeciesSpinel (synthetic)
VarietyFlame Fusion (Verneuil)
Blue VioletRed PinkColorlessGreenYellow 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

Synthetic Spinel reads 1.727 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, at 1.727, and it does not move as you rotate. MEANSSingly refractive. A stationary single edge through thewhole rotation is the observation that says so. 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

Synthetic Spinel Dark throughout (SR) 0° 45° 90° 135° ROTATION BETWEEN CROSSED POLARS Synthetic 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

Synthetic Spinel SG 3.61–3.66 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 14 other species in the table overlap this range, so a reading here narrows the field to 15 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

Synthetic Spinel One colour (no pleochroism) IDENTICAL AT EVERY ANGLE Synthetic 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.

Key Differentiators

Natural vs. Synthetic

Synthetic Spinel is commercially available (Flame fusion (Verneuil)). Distinguishing natural from synthetic typically requires microscopic examination of internal features.

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

Commonly Confused With

Commonly confused with: Sapphire, Ruby, Aquamarine, Emerald, Peridot, Alexandrite.

Treatments

Related Comparisons

Natural Vs Synthetic Spinel

Measurement Guides

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

Identifying a synthetic 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? Synthetic 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.

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