igneous
Vesicular basalt pebble
Basalt, a mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, commonly with olivine
AI-generated identification · may be incorrectDark gray to tan, fine-grained, dull to mildly vitreous, with abundant rounded vesicles (gas bubbles); hardness about 5–6 Mohs, no obvious cleavage, and typical specific gravity near 2.7–3.0. Its smooth rounded shape indicates water or beach abrasion, while the holes may be partly weathered or mineral-filled.
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Physical properties
Dark gray to tan, fine-grained, dull to mildly vitreous, with abundant rounded vesicles (gas bubbles); hardness about 5–6 Mohs, no obvious cleavage, and typical specific gravity near 2.7–3.0. Its smooth rounded shape indicates water or beach abrasion, while the holes may be partly weathered or mineral-filled.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, trapping volcanic gases; the specimen’s exact age cannot be determined visually and may range from ancient to modern. Common settings include lava flows, volcanic islands, river gravels, and beaches.
Uses & applications
Used as aggregate, road stone, riprap, landscaping material, and occasionally as a decorative or teaching specimen; rarely used in jewelry except as polished cabochons.
Geological facts
Vesicles are characteristic of gas-rich lava, but the unusually regular holes can also occur in weathered vesicular basalt or industrial slag. Basalt is the most abundant volcanic rock in Earth’s oceanic crust.
Field identification & locations
Test for a natural, fine-grained texture and relatively high density; a streak test is usually pale gray, and a magnet may show weak attraction. Common worldwide near volcanic terrains; avoid confusing it with porous concrete or slag, which may show glassy patches, metallic grains, or unnaturally sharp cavities.
