igneous
Rhyolite with Agate/Quartz Vugs
Rhyolite (Volcanic Rock) containing SiO2 (Silica) vugs
Hardness: 6-7 (Mohs scale); Color: Reddish-brown, gray, or pinkish matrix with white/translucent crystal cavities; Luster: Dull to earthy matrix with vitreous (glassy) crystal centers; Structure: Fine-grained aphanitic or porphyritic matrix with vugs (cavities).
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull to earthy matrix with vitreous (glassy) crystal centers
Identified More igneous →
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Reddish-brown, gray, or pinkish matrix with white/translucent crystal cavities; Luster: Dull to earthy matrix with vitreous (glassy) crystal centers; Structure: Fine-grained aphanitic or porphyritic matrix with vugs (cavities).
Formation & geological history
Formed from the rapid cooling of silica-rich lava at or near the Earth's surface. The cavities (vugs) were formed by gas bubbles trapped in the cooling lava, which were later filled or lined with microcrystalline quartz (agate) or macrocrystalline quartz from hydrothermal fluids.
Uses & applications
Used primarily as decorative landscaping stones, decorative specimens for collectors, and sometimes in construction aggregate or as a source material for small lapidary projects.
Geological facts
Rocks like this are often referred to as 'Thunder Eggs' before they are cut and polished if they form in specific spherical nodules. They are common in volcanic regions like the American West (Oregon, Nevada, and Utah).
Field identification & locations
Identify in the field by looking for a fine-grained, light-colored volcanic rock matrix containing holes or pockets filled with glassy minerals. Common in Cenozoic volcanic fields.
