Basalt

Physical Properties

Hardness: 6.0 on Mohs scale; Color: Black, dark gray, or dark green; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None; Specific Gravity: 2.8-3.0.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. It is the most common volcanic rock and makes up much of the ocean floor.

Uses & Applications

Used extensively in construction as crushed stone for road base, railroad ballast, and concrete aggregate. High-quality specimens are used for floor tiles, monuments, and as 'hot stones' for massage therapy.

Geological Facts

Basalt is also found on other planetary bodies, including the Moon (forming the dark 'maria' or seas), Mars, and Venus. It is the most common rock type in the Earth's crust.

Field Identification & Locations

Identify in the field by its dark color, heavy weight (relative to size), and very fine-grained texture. It often weathers to a brown or red color on the surface due to iron oxidation.

Identified on: 4/19/2026

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