Fire Opal

Related: Opal Varieties

Black OpalOpal

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.

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 4.12–4.20 (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 1.619–1.627 (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)

About Fire Opal

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.