igneous
Tumbled basalt pebble
Basalt, a mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, and dull to subtly glassy; hardness about 5–6 Mohs, massive with no visible cleavage, and specific gravity roughly 2.8–3.0. The smooth rounded surface indicates natural abrasion or tumbling; rusty-brown areas likely iron oxidation or weathered material.
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Physical properties
Typically dark gray to black, fine-grained, and dull to subtly glassy; hardness about 5–6 Mohs, massive with no visible cleavage, and specific gravity roughly 2.8–3.0. The smooth rounded surface indicates natural abrasion or tumbling; rusty-brown areas likely iron oxidation or weathered material.
Formation & geological history
Forms when low-silica basaltic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and flood-basalt provinces. The specimen’s age cannot be determined from the photograph and may range from recent to billions of years.
Uses & applications
Used widely as crushed aggregate, road stone, riprap, railroad ballast, and occasionally architectural stone; polished pebbles are sold for landscaping and collections.
Geological facts
Basalt is Earth’s most abundant volcanic rock and makes up much of the ocean floor. Vesicles may occur, but this example appears largely massive and fine-grained.
Field identification & locations
Identify it by its heavy feel, dark fine-grained texture, lack of obvious individual crystals, and resistance to scratching; a steel file may produce a faint mark. Common sources include volcanic regions, river gravels, beaches, and glacial deposits.
