igneous
Basalt
Basalt (comprised primarily of plagioclase, pyroxene, and olivine)
Hardness: 6 (Mohs scale), Color: Dark grey to black, Luster: Dull/earthy, Crystal structure: Aphanitic (fine-grained), Specific gravity: 2.8–3.0
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Physical properties
Hardness: 6 (Mohs scale), Color: Dark grey to black, Luster: Dull/earthy, Crystal structure: Aphanitic (fine-grained), Specific gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. It is the most common volcanic rock on Earth.
Uses & applications
Used in construction as a base for roads, railroad ballast, and in concrete aggregate. It is also used as dimension stone for tiles and counter tops, and sometimes as a healing stone in spa treatments.
Geological facts
Basalt makes up more than 90% of all volcanic rock on Earth and underlies most of the Earth's oceans. The Giant's Causeway in Northern Ireland is a famous example of columnar basalt.
Field identification & locations
Identify by its dark color, heavy weight compared to other small stones, and very fine grains that are hard to see without magnification. Found in volcanic regions and ocean floors globally.
