← Back to Reference Database RI Chart SG Chart Fluorescence Hardness

Fire Opal

Last updated: April 2026

Fire opal is the orange-to-red variety of opal from Mexico (principally Querétaro state), typically lacking play-of-color. Unlike precious opal, it is usually faceted. A synthetic version (Mexifire, by Kyocera) has been documented (GIA Fall 2008) — check for too-perfect body color uniformity and absence of any natural inclusions. Hydrophane character (absorbs water) is absent in most fire opal, distinguishing it from Ethiopian Welo opal.

Fire Opal has a specific gravity of 2.00 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 Range1.98–2.20
SG Typical2.00
Hardness (Mohs)5.5–6.5
Crystal SystemAmorphous
Optic CharacterSR (Singly Refractive)
Fluorescence LWVariable
Fluorescence SWInert
Chelsea FilterNot Applicable
PleochroismNone
ColorsYellow Orange, Red Pink
SpeciesOpal
Yellow OrangeRed Pink

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

Fire 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

Fire Opal Dark throughout (SR) 0° 45° 90° 135° ROTATION BETWEEN CROSSED POLARS Fire 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

Fire Opal SG 1.98–2.20 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. Fire 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

Fire Opal One colour (no pleochroism) IDENTICAL AT EVERY ANGLE Fire 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 OpalOpal

Key Differentiators

Natural vs. Synthetic

Synthetic fire 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 Fire Opal.

Common Simulants

Treatments

Measurement Guides

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

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

Try Loupewise Free →

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