igneous
Polished porphyritic volcanic rock (likely andesite or basalt)
Fine-grained mafic/intermediate igneous rock; likely basaltic-andesite/andesite with feldspar phenocrysts
AI-generated identification · may be incorrectDark brown to black, rounded and polished, with pale gray-white feldspar crystals or weathered mineral-filled vesicles; generally fine-grained, massive, Mohs hardness about 5–6, dull to slightly vitreous luster, and no obvious cleavage.
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Physical properties
Dark brown to black, rounded and polished, with pale gray-white feldspar crystals or weathered mineral-filled vesicles; generally fine-grained, massive, Mohs hardness about 5–6, dull to slightly vitreous luster, and no obvious cleavage.
Formation & geological history
Formed when lava cooled relatively quickly at or near Earth’s surface; its exact geological age cannot be determined from the photograph. Common in volcanic terrains and river or beach gravels where abrasion rounded it.
Uses & applications
Used as decorative stone, aggregate, landscaping rock, and a collectible specimen; rarely suitable as a gemstone. A polished example may have modest aesthetic value.
Geological facts
The pale markings may be feldspar phenocrysts, amygdules, or weathered cavities rather than fossils. Confirming the name requires a fresh surface, hand lens, streak, magnetism, and ideally thin-section or chemical analysis.
Field identification & locations
Look for a dense dark groundmass, scattered light crystals, and possible vesicles; basalt is typically darker and more iron-rich, while andesite is commonly gray-brown with abundant feldspar. Volcanic rocks occur worldwide, especially around subduction zones and oceanic islands.
