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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
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.

Through the polariscope

Coral Light throughout (AGG) 0° 45° 90° 135° ROTATION BETWEEN CROSSED POLARS Coral stays light in every position. Light crosses many randomly oriented grains, so some component always reaches the analyser. On a refractometer an aggregate is indistinguishable from a singly refractive stone; this is the test that separates them. 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

Coral SG 2.60–2.70 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 36 other species in the table overlap this range, so a reading here narrows the field to 37 rather than naming the stone. This is the crowded part of the scale — the same careful measurement is worth far more higher up. Coral is porous: it soaks up water during immersion, which raises the weight in water and makes the SG read HIGH. Heavy liquids damage porous material, so there is no clean fallback — treat any figure here as unreliable. 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

Coral One colour (no pleochroism) IDENTICAL AT EVERY ANGLE Coral shows no pleochroism, so both windows match at every orientation — turning the instrument changes nothing. That is a real negative result, and the reason it is worth knowing is below. 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.

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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