igneous
Dark fine-grained stone, likely basalt
Basalt; chiefly plagioclase feldspar and pyroxene, commonly with minor olivine and iron oxides
AI-generated identification · may be incorrectUsually dark gray to black, fine-grained, dull to subtly glassy, with no visible large crystals; hardness about 6, nonfoliated, and typically dense (specific gravity ~2.8–3.0). The fractured surface and shape are also consistent with a basalt flake or toolstone, though a photograph cannot confirm the identification.
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Physical properties
Usually dark gray to black, fine-grained, dull to subtly glassy, with no visible large crystals; hardness about 6, nonfoliated, and typically dense (specific gravity ~2.8–3.0). The fractured surface and shape are also consistent with a basalt flake or toolstone, though a photograph cannot confirm the identification.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at Earth’s surface, commonly in volcanic flows, oceanic crust, and flood-basalt provinces. It may range from recent to Precambrian in age, depending on locality.
Uses & applications
Used extensively as crushed aggregate, road stone, riprap, paving, and construction material; fine-quality pieces are collected, and suitable basalt can be knapped into tools.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its fine texture reflects rapid cooling, which prevented large crystals from forming.
Field identification & locations
Look for a uniformly dark, heavy, fine-grained surface that does not fizz with dilute hydrochloric acid and is harder than a steel knife. Common localities include Hawaii, Iceland, oceanic islands, and volcanic regions worldwide.
