igneous
Likely porphyritic basalt or andesite fragment
Fine-grained mafic to intermediate volcanic rock; commonly plagioclase ± pyroxene, amphibole, and iron oxides
AI-generated identification · may be incorrectDark gray-purple, fine-grained matrix with contrasting pale green-gray mineral patches; typically dull to slightly glassy, hardness about 5–6, massive, with no obvious cleavage. Exact identification is limited by the small, weathered specimen and image quality.
Identified More igneous →
Explore Likely porphyritic basalt or andesite fragment in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Dark gray-purple, fine-grained matrix with contrasting pale green-gray mineral patches; typically dull to slightly glassy, hardness about 5–6, massive, with no obvious cleavage. Exact identification is limited by the small, weathered specimen and image quality.
Formation & geological history
Formed when lava cooled rapidly at or near Earth’s surface, producing a fine matrix around larger crystals; likely Cenozoic or older, depending on source locality. Common in volcanic terrains and as transported gravel or construction aggregate.
Uses & applications
Used mainly as crushed stone, road aggregate, riprap, and occasionally as an ornamental stone; this small fragment has negligible commercial value.
Geological facts
The greenish areas may represent altered pyroxene or amphibole, while pale grains may be plagioclase. A fresh surface, streak, density, and a hand lens would improve identification.
Field identification & locations
Look for a dense, dark volcanic rock with tiny interlocking crystals and possible pale feldspar or green alteration minerals; common worldwide in lava flows and volcanic uplands. Estimated value: $0–$5 as an ordinary fragment, or about $5–$20 if locality-documented and visually attractive.
