igneous
Rhyolite (possibly porphyritic and altered)
Volcanic felsic rock; primarily quartz, alkali feldspar, plagioclase, and volcanic glass, with iron-oxide alteration
AI-generated identification · may be incorrectHardness about 6–7; mottled gray, tan, reddish-brown, and black; generally dull to slightly vitreous; fine-grained groundmass with possible porphyritic crystals and irregular fracture. Exact mineralogy is uncertain from the image.
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Physical properties
Hardness about 6–7; mottled gray, tan, reddish-brown, and black; generally dull to slightly vitreous; fine-grained groundmass with possible porphyritic crystals and irregular fracture. Exact mineralogy is uncertain from the image.
Formation & geological history
Forms when silica-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic domes and flows. Specimen age depends on locality, ranging from Precambrian to recent volcanic provinces.
Uses & applications
Used locally as aggregate, road stone, and building stone; attractive patterned pieces are collected or cut for decorative objects, though this specimen has modest gem value.
Geological facts
Rhyolite is the volcanic equivalent of granite and may contain flow banding, welded textures, or cavities filled by secondary minerals such as quartz or chalcedony.
Field identification & locations
Identify by its very fine texture, light felsic base, scattered crystals, and volcanic banding or brecciation; common in Yellowstone, the western United States, Mexico, Europe, and volcanic belts worldwide.
