igneous
Banded basalt pebble with a quartz-rich vein
Basalt, likely amygdaloidal or hydrothermally altered; chiefly plagioclase feldspar and pyroxene, with quartz/calcite and iron oxides
AI-generated identification · may be incorrectDark gray, fine-grained, massive pebble with pale horizontal bands and a white crystalline cap/vein; typically Mohs 5–6, dull to subvitreous luster, very fine interlocking crystals, and no obvious cleavage. The rusty areas are iron-oxide staining or weathered sulfides.
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Physical properties
Dark gray, fine-grained, massive pebble with pale horizontal bands and a white crystalline cap/vein; typically Mohs 5–6, dull to subvitreous luster, very fine interlocking crystals, and no obvious cleavage. The rusty areas are iron-oxide staining or weathered sulfides.
Formation & geological history
Formed from rapidly cooled basaltic lava, probably later fractured and filled or coated by silica- or carbonate-rich fluids; the exact geological age cannot be determined from the photograph. Common in volcanic terrains and stream or beach gravels.
Uses & applications
Mostly a teaching or ornamental/collector specimen; basalt is widely used as aggregate, road stone, riprap, and dimension stone, while quartz-bearing portions may be polished.
Geological facts
The regular pale bands may represent amygdules, alteration zones, or layered lava textures rather than true sedimentary bedding. A white cap could be quartz, calcite, or weathered feldspar and requires testing for confident identification.
Field identification & locations
Check for tiny feldspar crystals, a dense heavy feel, and weak magnetism; test the white material with vinegar or dilute acid for calcite, and glass for hardness. Common in Iceland, India, the Pacific Northwest, Hawaii, and other volcanic regions.
