igneous
Polished porphyritic rock, likely porphyritic basalt or andesite
Porphyritic volcanic rock; exact identification requires examination of fresh surface and mineral tests
AI-generated identification · may be incorrectDark gray-brown, rounded and polished, with pale mineral crystals or speckles in a finer-grained groundmass. Likely hardness 5–6; crystals are too small to identify reliably from the photo.
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Physical properties
Dark gray-brown, rounded and polished, with pale mineral crystals or speckles in a finer-grained groundmass. Likely hardness 5–6; crystals are too small to identify reliably from the photo.
Formation & geological history
Formed when mineral crystals began growing in magma before the remaining melt cooled rapidly at or near Earth’s surface. Its age and specific volcanic setting cannot be determined from the image.
Uses & applications
Likely a polished collecting stone or worry stone; porphyritic volcanic rocks are also used as aggregate and, when suitable, decorative stone.
Geological facts
The contrasting larger crystals and fine groundmass are called a porphyritic texture, recording two stages of cooling.
Field identification & locations
A dark fine-grained matrix with conspicuous light grains suggests porphyritic volcanic rock; a fresh broken face and mineral identification are needed to distinguish basalt from andesite. Similar rocks occur in volcanic regions worldwide.
