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Opal

Last updated: April 2026

Precious opal exhibits play-of-color from microscopic silica sphere diffraction. Common opal (potch) lacks play-of-color and appears milky, opaque, or translucent in a single body color — common opal is often misidentified as glass, chalcedony, or moonstone. If no play-of-color is visible, consider common opal or another gem species entirely. Synthetic producers include Gilson (France) and Kyocera (Japan, 'Crescent Shire'). Ethiopian Welo opals are hydrophane and may absorb oil, altering play-of-color; check by submersion in water (reversible enhancement).

Opal has a specific gravity of 2.15 and a refractive index of 1.370–1.470, with a Mohs hardness of 5.5–6.5. It is singly refractive.

Physical & Optical Properties

RI Range1.370–1.470
SG Range1.98–2.25
SG Typical2.15
Hardness (Mohs)5.5–6.5
Crystal SystemAmorphous
Optic CharacterSR (Singly Refractive)
Dispersion0.020
Fluorescence LWVariable
Fluorescence SWVariable
Chelsea FilterInert
PleochroismNone
ColorsColorless, Yellow Orange, Red Pink, Green, Blue Violet, Black, Multi
SpeciesOpal
ColorlessYellow OrangeRed PinkGreenBlue VioletBlackMulti

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

Opal varies 1.370 – 1.470 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 1.370 –1.470, and the stone drawn here reads 1.420 — the dashedmark is the one end of that range this scale can show. MEANSSingly refractive — a stationary single edge through thewhole rotation says so. The value itself is only anarrowing: this species spans 10 printed divisions, so areading places your stone inside the range rather thannaming it. The bottom of that range is under this scale's1.40 floor, so the instrument cannot place every stone ofthis species. 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

Opal Dark throughout (SR) 0° 45° 90° 135° ROTATION BETWEEN CROSSED POLARS Opal stays dark through the whole rotation. That is singly refractive: there is no second ray to rotate the plane of vibration back into the analyser. 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

Opal SG 1.98–2.25 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 5 other species in the table overlap this range, so a reading here narrows the field to 6 rather than naming the stone. Combine it with a refractive index to separate what is left. Opal 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

Opal One colour (no pleochroism) IDENTICAL AT EVERY ANGLE Opal 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.

Black OpalFire Opal

Key Differentiators

Natural vs. Synthetic

Synthetic opal is commercially available (Gilson synthetic (columnar growth)). Distinguishing natural from synthetic typically requires microscopic examination of internal features.

Loupewise Pro includes a two-phase natural vs. synthetic testing protocol for Opal.

Treatments

Measurement Guides

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

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

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Studying for the GIA, FGA, or FEEG practical? Opal 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 →