Basalt

Physical Properties

Hardness: 6-7 Mohs; Color: dark grey to black (weathered to brown); Luster: dull/earthy; Crystal structure: fine-grained or aphanitic; Specific gravity: 2.8-3.0

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. This specimen appears water-worn, likely tumbled in a river or creek bed.

Uses & Applications

Used extensively in construction for aggregate, asphalt, and railroad ballast. Exceptionally durable for road bases.

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.

Field Identification & Locations

Identify in the field by its dark color, heavy weight for its size, and microscopic grain size. Found in volcanic regions and riverbeds carrying volcanic runoff.

Identified on: 4/16/2026

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