igneous
Rhyolite or fine-grained volcanic rock
Rhyolite ( felsic volcanic rock; commonly quartz + alkali feldspar)
AI-generated identification · may be incorrectGray-brown, fine-grained, dull to subtly silky surface, with no visible large crystals; estimated Mohs hardness 6–7, low porosity, and conchoidal to irregular fracture. The tapered shape appears naturally weathered or manually shaped, so exact identification from the image is uncertain.
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Physical properties
Gray-brown, fine-grained, dull to subtly silky surface, with no visible large crystals; estimated Mohs hardness 6–7, low porosity, and conchoidal to irregular fracture. The tapered shape appears naturally weathered or manually shaped, so exact identification from the image is uncertain.
Formation & geological history
Forms when silica-rich lava cools rapidly at or near Earth’s surface, commonly during volcanic eruptions; specimens range from Archean to recent age. Its fine texture reflects rapid cooling compared with granite.
Uses & applications
Used locally as crushed aggregate, road stone, fill, and occasionally as a lapidary or ornamental material; this unpolished specimen has little commercial value.
Geological facts
Rhyolite is the volcanic equivalent of granite and may contain microscopic quartz and feldspar crystals. Similar-looking materials include weathered basalt, chert, and fine-grained quartzite.
Field identification & locations
Check for a uniformly fine texture, light gray to pink-brown color, hardness sufficient to scratch glass, and lack of strong layering or foliation; a streak, acid, and hand-lens test would help distinguish it. Common in volcanic regions worldwide, including the western United States, Iceland, and New Zealand.
