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Black Spinel

Last updated: April 2026

A hard, durable, and singly refractive gemstone often used as a simulant for black diamonds.

Black Spinel has a specific gravity of 3.60 and a refractive index of 1.712–1.730, with a Mohs hardness of 8. It is singly refractive.

Physical & Optical Properties

RI Range1.712–1.730
SG Range3.58–3.61
SG Typical3.60
Hardness (Mohs)8
Crystal SystemCubic
Optic CharacterSR (Singly Refractive)
Dispersion0.020
Fluorescence LWInert
Fluorescence SWInert
Chelsea FilterInert
PleochroismNone
ColorsBlack
SpeciesSpinel
Black

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

Black Spinel varies 1.712 – 1.730 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 –1.730, and the stone drawn here reads 1.721, 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

Black Spinel No reading ANY POSITION — THE STONE BLOCKS THE LIGHT BETWEEN CROSSED POLARS Black Spinel is opaque, so no light reaches the analyser. The field stays dark in every position — not because the stone is singly refractive, but because the test cannot be run on it at all. SR, DR and aggregate cannot be separated this way. 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

Black Spinel SG 3.58–3.61 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 10 other species in the table overlap this range, so a reading here narrows the field to 11 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

Black Spinel No reading NO LIGHT THROUGH THE STONE Black Spinel is opaque. A dichroscope reads light that has passed THROUGH the stone, so there is nothing to split and no reading to take — the same limit that rules out the polariscope. 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.

Spinel

Key Differentiators

Natural vs. Synthetic

Synthetic black 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 Black Spinel.

Common Simulants

Commonly Confused With

Commonly confused with: Black Diamond, Black Onyx, Black Tourmaline.

Treatments

Measurement Guides

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

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

Try Loupewise Free →

Studying for the GIA, FGA, or FEEG practical? Black 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 →