igneous
Weathered vesicular volcanic rock (likely basalt or andesite)
Amygdaloidal basalt/andesite; cavities may contain calcite, zeolites, or silica minerals
AI-generated identification · may be incorrectDark brown, fine-grained, dull to slightly waxy surface with rounded vesicles and pale mineral-filled cavities; estimated Mohs hardness 5–6, with no obvious cleavage. Exact identification requires a fresh surface, streak, density, and mineral testing.
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Physical properties
Dark brown, fine-grained, dull to slightly waxy surface with rounded vesicles and pale mineral-filled cavities; estimated Mohs hardness 5–6, with no obvious cleavage. Exact identification requires a fresh surface, streak, density, and mineral testing.
Formation & geological history
Formed when gas-rich lava cooled rapidly, trapping bubbles later partly filled by secondary minerals; likely Cenozoic or older, but age cannot be determined visually. Brown coloration reflects surface weathering and iron oxidation.
Uses & applications
Primarily a collector specimen or decorative stone; fresh basalt and andesite are used as aggregate, roadstone, and building material, while zeolite- or calcite-filled pieces may be ornamental.
Geological facts
The cavities are called vesicles; mineral-filled vesicles are termed amygdules. The rounded shape and smooth brown rind indicate prolonged transport or weathering, possibly as a river pebble.
Field identification & locations
Identify by its fine volcanic texture, irregular holes, hard dark matrix, and pale cavity fillings; common in volcanic regions and stream gravels worldwide. A simple acid test may reveal calcite fillings, while a steel file tests hardness.
