igneous
Weathered rhyolite (possibly porphyritic volcanic rock)
Rhyolite; chiefly quartz and alkali feldspar, with minor plagioclase and iron-bearing alteration minerals
AI-generated identification · may be incorrectHardness about 6–7 Mohs; pale gray, cream, pink, and greenish weathered colors with dark streaks; generally dull to slightly vitreous luster. Fine-grained volcanic groundmass and possible faint porphyritic texture are visible, though heavy weathering obscures the crystals; no reliable cleavage as a rock.
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Physical properties
Hardness about 6–7 Mohs; pale gray, cream, pink, and greenish weathered colors with dark streaks; generally dull to slightly vitreous luster. Fine-grained volcanic groundmass and possible faint porphyritic texture are visible, though heavy weathering obscures the crystals; no reliable cleavage as a rock.
Formation & geological history
An extrusive felsic igneous rock formed when silica-rich lava cooled rapidly at or near Earth’s surface, commonly during volcanic eruptions from the Precambrian to Recent. The rusty and green colors likely reflect surface weathering and alteration.
Uses & applications
Used locally as crushed aggregate, road stone, landscaping stone, and occasionally as an ornamental or carving material; ordinary specimens have little gem value.
Geological facts
Rhyolite is the volcanic equivalent of granite and can contain quartz-rich flow banding or lithophysae. This specimen’s rounded, weathered form suggests transport or prolonged surface exposure.
Field identification & locations
Look for pale color, fine grain, quartz/feldspar-rich composition, and flow banding; distinguish it from granite by the lack of clearly visible large crystals. Common in volcanic terrains worldwide, including the western United States, Mexico, Iceland, and New Zealand.
