Black Spinel

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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

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: RI ~1.718–1.725 for natural black spinel. Synthetic: Flame fusion synthetic spinel RI ~1.727 — distinctly higher. Quick refractometer check is a useful indicator.
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.

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

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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 1.530–1.540; SG ~2.65; much lower hardness (6.5–7 Mohs).

Commonly Confused With

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

Treatments

Surface Coating (to enhance luster or color)
Fracture Filling (resin/glass)

Price Context

Natural — low ($/ct)$20

Natural — high ($/ct)$150

NotePer carat; popular diamond and onyx alternative; opaque black

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

Measurement Guides

Identifying a black 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? Black 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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