igneous
Rhyolite (likely porphyritic, iron-stained)
Rhyolite; fine-grained volcanic rock dominated by quartz and alkali feldspar
AI-generated identification · may be incorrectUsually hardness 6–7, pink/red to gray, with dull to slightly vitreous luster; commonly contains tiny feldspar or quartz phenocrysts in a fine groundmass. The reddish coloration is consistent with iron-oxide staining or oxidized volcanic minerals.
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Physical properties
Usually hardness 6–7, pink/red to gray, with dull to slightly vitreous luster; commonly contains tiny feldspar or quartz phenocrysts in a fine groundmass. The reddish coloration is consistent with iron-oxide staining or oxidized volcanic minerals.
Formation & geological history
Formed when silica-rich lava cooled rapidly at or near Earth’s surface, commonly during volcanic eruptions. Exact age and locality cannot be determined from the photograph; rhyolite ranges from Precambrian to modern volcanic settings.
Uses & applications
Used as crushed aggregate, roadstone, riprap, and occasionally decorative stone; attractive pieces may be collected but are rarely gem-quality.
Geological facts
Rhyolite is the volcanic equivalent of granite and may display flow banding, spherulites, or lithophysae. The specimen appears weathered and may alternatively represent a closely related felsite or welded volcanic rock.
Field identification & locations
Look for a very fine texture, pale quartz/feldspar specks, irregular fracture, and possible flow bands; it is common in continental volcanic provinces such as the western United States, Mexico, and New Zealand. A hand lens and thin-section test would improve identification.
