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
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
- Hardness of 8
- Vitreous luster
Natural vs. Synthetic
Synthetic black spinel is commercially available (Flame fusion (Verneuil)). Distinguishing natural from synthetic typically requires microscopic examination of internal features.
- RI: This reference records natural black spinel at RI 1.712–1.730 — a wider band than the “~1.718” often quoted, and it CONTAINS the figure usually given for flame-fusion synthetic. Synthetic: Flame-fusion synthetic spinel is commonly quoted at about 1.727 and above, from excess Al₂O₃ in the melt. That value sits inside the natural range above, so an RI reading confirms the species and cannot settle the origin. Use the polariscope strain pattern and magnification instead.
- Polariscope (ADR): Weak to moderate anomalous double refraction (ADR). Faint wavy or patchy extinction. Synthetic: Strong tabby extinction — checkerboard ADR pattern under crossed polarizers. Diagnostically useful for flame fusion.
- Microscopy: Natural inclusions: negative crystals (angular/hexagonal), irregular fluid inclusions, growth zoning. Synthetic: Curved growth striae and spherical/tadpole gas bubbles. Often in rows parallel to striae.
Loupewise Pro includes a two-phase natural vs. synthetic testing protocol for Black Spinel.
Common Simulants
- Black diamond: Diamond has extreme hardness (10 Mohs); thermal probe reacts strongly; RI much higher (~2.42); SG 3.52.
- Black onyx / chalcedony: Aggregate optic character; RI 1.530–1.543; SG 2.58–2.64; much lower hardness (6.5–7 Mohs).
Commonly Confused With
Commonly confused with: Black Diamond, Black Onyx, Black Tourmaline.
Treatments
- Surface Coating (to enhance luster or color)
- Fracture Filling (resin/glass)
Measurement Guides
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 →