Where is Fire Opal in Rhyolite found?
Opal (SiO2·nH2O) in Rhyolite
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Open the appFire Opal in Rhyolite occurs in specific geological environments. Knowing where to look, and in what kind of rock, is the difference between going home empty-handed and finding a great specimen.
Where to find Fire Opal in Rhyolite
Mexico
Mexico is the primary source of fire opal in rhyolite, particularly the states of Querétaro, Jalisco, Hidalgo, Michoacán, and Guerrero. The opal often occurs in 'cantera' (quarry) deposits within rhyolitic tuffs and lavas.
Oregon, USA
The Virgin Valley in Nevada and parts of Oregon are known for precious opal, sometimes found in volcanic matrices, though rhyolite-hosted fire opal is less common than in Mexico.
Ethiopia
Welo and Mezezo deposits produce significant amounts of precious opal, often found in volcanic rocks, including rhyolitic and trachytic tuffs.
Australia
While Australia is famous for opal, most Australian opal is sedimentary in origin. However, some volcanic-hosted opal can be found, though rhyolite-hosted fire opal is not a primary characteristic.
Collecting tips
- Look for Volcanic Terrains. Focus your search on areas with known rhyolitic volcanic activity, especially those with evidence of hydrothermal alteration.
- Examine Vugs and Fractures. Opal typically forms in open spaces. Look for cavities, vesicles, and fractures within the rhyolite matrix. These can be exposed by breaking open rocks or found on weathered surfaces.
- Identify Opal's Luster. The vitreous to resinous luster of opal will stand out against the duller, earthy luster of the rhyolite matrix. Look for the characteristic fiery colors.
- Check for Play-of-Color. If searching for precious fire opal, rotate the specimen under a light source to observe any iridescent flashes of color, which indicate play-of-color.
- Safety Precautions. When collecting in volcanic areas, be aware of unstable ground, sharp rocks, and potential for rockfalls. Always wear appropriate safety gear, including eye protection and gloves. Be mindful of silica dust if breaking rocks, as prolonged inhalation can lead to silicosis.
Quick Check
- Color:
- Light-colored rhyolite matrix (gray, pink, tan) with yellow, orange, or red opal infillings.
- Luster:
- Dull to earthy for rhyolite; vitreous to resinous for opal.
- Streak:
- White for both rhyolite (due to its felsic composition) and opal (amorphous silica).
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