mineral
Fire Opal (Common/Precious in Matrix)
Hydrated Silica (SiO2·nH2O)
Hardness: 5.5-6.5 (Mohs); Color: Orange, yellow, or white set in tan rhyolite matrix; Luster: Vitreous to waxy; Structure: Amorphous; Cleavage: None (Conchoidal fracture).
- Hardness
- 5
- Color
- Orange, yellow, or white set in tan rhyolite matrix
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 5.5-6.5 (Mohs); Color: Orange, yellow, or white set in tan rhyolite matrix; Luster: Vitreous to waxy; Structure: Amorphous; Cleavage: None (Conchoidal fracture).
Formation & geological history
Formed as silica-rich water seeps into cavities of volcanic rocks, typically rhyolite. Most deposits date from the Tertiary period (roughly 20-30 million years ago) specifically in high-altitude volcanic regions.
Uses & applications
Used primarily in the jewelry industry for cabochon cutting, as gemstones for collectors, and as decorative metaphysical specimens.
Geological facts
Unlike other opals, fire opals from Mexico and similar regions are valued specifically for their warm body color rather than just 'play of color'. They are amorphous, meaning they lack a crystal structure.
Field identification & locations
Identify in the field by looking for translucent, jelly-like orange or white nodes embedded in light-colored, porous volcanic rock. Common in Mexico (Querétaro), Ethiopia, and the Western USA.
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