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Obsidian

Last updated: April 2026

A naturally occurring volcanic glass formed when lava cools rapidly.

Obsidian has a specific gravity of 2.40 and a refractive index of 1.480–1.510, with a Mohs hardness of 5.0–5.5. It is singly refractive.

Physical & Optical Properties

RI Range1.480–1.510
SG Range2.30–2.60
SG Typical2.40
Hardness (Mohs)5–5.5
Crystal SystemAmorphous
Optic CharacterSR (Singly Refractive)
Dispersion0.010
Fluorescence LWInert
Fluorescence SWInert
Chelsea FilterInert
PleochroismNone
ColorsBlack, Brown, Green, Multi
SpeciesNatural Glass
BlackBrownGreenMulti

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

Obsidian varies 1.480 – 1.510 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 1.480 –1.510, and the stone drawn here reads 1.495, between thedashed marks. MEANSSingly refractive — a stationary single edge through thewhole rotation says so. The value itself is only anarrowing: this species spans 3 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.

Key Differentiators

Treatments

Measurement Guides

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

Identifying an obsidian? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.

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Studying for the GIA, FGA, or FEEG practical? Obsidian 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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