Basalt

Physical Properties

Hardness: 6; Color: Dark grey to black; Luster: Dull to submetallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific gravity: 2.8-3.0

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. Most basalt flows on Earth were formed during the Cenozoic era, though it can occur in any geological age.

Uses & Applications

Extensively used in construction for aggregate, road base, concrete, and railroad ballast. It is also used as a gemstone in specialized jewelry or as an industrial fiber (basalt fiber).

Geological Facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found on the Moon and Mars, forming the dark 'seas' known as lunar maria.

Field Identification & Locations

Identified in the field by its dark color, heavy weight (relative to size), and fine-grained texture lacking visible crystals. It can often be found near volcanic active regions or oceanic crust outcrops.

Identified on: 5/16/2026

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