igneous
Orbicular rhyolite (likely)
Rhyolite, a silica-rich volcanic rock; commonly quartz, alkali feldspar, plagioclase, and iron-rich alteration minerals
AI-generated identification · may be incorrectFine-grained, pale gray to reddish-brown matrix with abundant pale, elongated feldspar crystals or spherulitic inclusions; vitreous-to-dull luster, no obvious cleavage as a rock, and typical hardness about 6–7 Mohs. The rounded shape appears tumbled or naturally water-worn.
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Physical properties
Fine-grained, pale gray to reddish-brown matrix with abundant pale, elongated feldspar crystals or spherulitic inclusions; vitreous-to-dull luster, no obvious cleavage as a rock, and typical hardness about 6–7 Mohs. The rounded shape appears tumbled or naturally water-worn.
Formation & geological history
Formed when high-silica lava cooled rapidly at or near Earth’s surface, trapping crystals and producing a porphyritic texture; likely Cenozoic or younger, though age cannot be determined visually. Common in volcanic terrains and hydrothermal regions.
Uses & applications
Used mainly as a lapidary, ornamental, and collector stone; occasionally as decorative aggregate or facing material. Its value is aesthetic rather than industrial.
Geological facts
The pale markings may be feldspar phenocrysts, flow-banded material, or devitrified volcanic glass; exact identification requires a fresh surface or thin section. It resembles some porphyritic dacites and rhyolites.
Field identification & locations
Look for a fine volcanic groundmass, scattered light feldspar crystals, reddish iron staining, and lack of sedimentary layering. Common sources include western North America, Mexico, New Zealand, and other volcanic provinces.
