igneous
Likely polished rhyolite or porphyritic volcanic rock
Rhyolite, possibly flow-banded or altered rhyolite; composition mainly quartz and alkali feldspar with minor biotite/iron oxides
AI-generated identification · may be incorrectHardness about 6–7 Mohs; rounded and polished, with bluish-gray groundmass, pale streaks, and yellow-brown mineral-rich bands. Typically fine-grained to porphyritic, with a dull to slightly vitreous luster and no obvious cleavage.
Identified More igneous →
Explore Likely polished rhyolite or porphyritic volcanic rock in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness about 6–7 Mohs; rounded and polished, with bluish-gray groundmass, pale streaks, and yellow-brown mineral-rich bands. Typically fine-grained to porphyritic, with a dull to slightly vitreous luster and no obvious cleavage.
Formation & geological history
An extrusive felsic igneous rock formed from viscous, silica-rich lava that cooled at or near the surface, commonly producing flow banding; ages range from Precambrian to recent volcanic settings.
Uses & applications
Used as decorative stone, lapidary material, landscaping rock, and occasionally aggregate; attractive polished pieces have modest collector value.
Geological facts
The specimen’s rounded shape indicates tumbling or natural abrasion, while the banded appearance may reflect welded volcanic textures or hydrothermal alteration.
Field identification & locations
Look for fine-grained texture, pale silica-rich bands, and hardness sufficient to scratch glass; identification would be improved by a fresh surface, streak test, and examination with a hand lens. Common in volcanic regions worldwide, including the western United States, Mexico, and New Zealand.
