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Coral

Last updated: April 2026

An organic gemstone formed from the skeletal remains of marine polyps.

Coral has a specific gravity of 2.68 and a refractive index of 1.486–1.658, with a Mohs hardness of 3.0–4.0. It is aggregate.

Physical & Optical Properties

RI Range1.486–1.658
SG Range2.60–2.70
SG Typical2.68
Hardness (Mohs)3–4
Crystal SystemAggregate
Optic CharacterAggregate
Dispersion0.000
Fluorescence LWInert
Fluorescence SWInert
Chelsea FilterInert
PleochroismNone
ColorsRed Pink, Colorless, Black, Blue Violet
SpeciesOrganic
Red PinkColorlessBlackBlue Violet

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

Coral varies 1.486 – 1.658 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 — and it isoften vague rather than crisp. Where it sits is not fixedfor this species: published at 1.486 – 1.658, and the stonedrawn here reads 1.572, between the dashed marks. MEANSThis is an aggregate — a mass of microscopic crystals — andon this instrument it looks exactly like a singly refractivestone. The refractometer cannot tell the two apart. Thepolariscope can: an aggregate stays light through a fullrotation, where a singly refractive stone stays dark. 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.

AmberIvoryPearl

Key Differentiators

Treatments

Measurement Guides

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

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

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