Mineraloid
Fire Opal (in matrix)
Hydrated Silica (SiO2·nH2O)
Hardness: 5.5-6.5 Mohs. Color: Vivid orange core (play-of-color) within a dark grey or basaltic matrix. Luster: Vitreous to sub-vitreous. Crystal structure: Amorphous. Specific gravity: 1.9-2.3.
- Hardness
- 5
- Luster
- Vitreous to sub-vitreous
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Physical properties
Hardness: 5.5-6.5 Mohs. Color: Vivid orange core (play-of-color) within a dark grey or basaltic matrix. Luster: Vitreous to sub-vitreous. Crystal structure: Amorphous. Specific gravity: 1.9-2.3.
Formation & geological history
Formed from silica-rich water seeping into cavities in volcanic rock (usually rhyolite or basalt) millions of years ago, often during the Tertiary period. The microscopic silica spheres diffract light to create the internal 'fire'.
Uses & applications
Primary use is in jewelry as gemstones (cabochons or rough settings) and as a lapidary material for mineral collectors.
Geological facts
Unlike 'Precious Opal', Mexican Fire Opal is valued specifically for its intense background body color (orange/red). It is the national gemstone of Mexico and was revered by the Aztecs who called it 'quetzalitzlipyollitli'.
Field identification & locations
Identify in the field by looking for glassy, translucent orange nodules embedded in weathered volcanic rock. Common locations include Queretaro and Jalisco in Mexico, as well as parts of Ethiopia and Australia.
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