igneous
Tumbled vesicular basalt (likely weathered basalt pebble)
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, with possible olivine and secondary zeolite/calcite minerals
AI-generated identification · may be incorrectDark gray to brown-gray, dull to slightly waxy luster, fine-grained and dense except for rounded vesicles; hardness about 5–6 Mohs, no obvious cleavage, typical specific gravity 2.8–3.0. The smooth shape indicates water tumbling or abrasion, while cavities reflect gas bubbles or later mineral dissolution.
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Physical properties
Dark gray to brown-gray, dull to slightly waxy luster, fine-grained and dense except for rounded vesicles; hardness about 5–6 Mohs, no obvious cleavage, typical specific gravity 2.8–3.0. The smooth shape indicates water tumbling or abrasion, while cavities reflect gas bubbles or later mineral dissolution.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, commonly in volcanic flows; its exact geological age cannot be determined from the image. Weathering and transport produced the rounded pebble and enlarged some vesicles.
Uses & applications
Used as aggregate, road stone, riprap, landscaping rock, and occasionally as a decorative or lapidary material; this specimen has little jewelry value.
Geological facts
Basalt is the most widespread volcanic rock on Earth and forms much of the oceanic crust. Vesicles may later fill with zeolites, quartz, or calcite, creating amygdaloidal basalt.
Field identification & locations
Identify by its heavy feel, dark fine-grained matrix, rounded weathering, and gas cavities; common in volcanic regions and river or beach gravels. A precise identification would benefit from a fresh surface, streak, and hardness testing.
