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
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
Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.
Related: Opal Varieties
Key Differentiators
- Orange-red body color with no play-of-color (typically)
- Low RI 1.37–1.47 — among the lowest of any gem mineral
- Low SG ~2.0 — noticeably light in hand
- Amorphous — singly refractive; no birefringence
- Mexican origin almost universal in trade material
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
- Carnelian / Red Chalcedony: Orange-red chalcedony, opaque to translucent. No play-of-color. RI 1.535–1.539 (higher than fire opal ~1.37–1.47); SG ~2.61 (heavier than fire opal ~2.0). Lacks the vitreous internal glow of fire opal.
- Spessartite Garnet: Orange to orange-red garnet; isotropic. SG 4.12–4.18 (much heavier than fire opal ~2.0); RI ~1.79–1.81 (much higher). No play-of-color; orange color may resemble faceted fire opal.
- Orange / Imperial Topaz: Biaxial positive; perfect basal cleavage visible under magnification. RI 1.609–1.643 (much higher than fire opal); SG 3.53 (heavier). Orange imperial topaz color may resemble faceted fire opal.
Treatments
- Resin or plastic impregnation (porous material stabilization)
- Smoke treatment (darkens body for play-of-color enhancement)
Measurement Guides
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 →