Basalt

Physical Properties

Hardness: 6 on Mohs scale; Color: dark grey to black, sometimes greenish; Luster: dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.8–3.0.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava (basaltic lava) exposed at or very near the surface. It is the most common rock type in the Earth's crust, forming the bulk of the ocean floor.

Uses & Applications

Primarily used in construction as aggregate (crushed stone), for concrete, and as road base material. High-quality varieties are used for floor tiles and monument stone.

Geological Facts

Basalt is also common on other planetary bodies; much of the Moon's 'maria' (dark plains) and large parts of Mars and Venus are composed of basaltic rock.

Field Identification & Locations

Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture with tiny holes (vesicles) sometimes present. Common near volcanic sites or in glacial till.

Identified on: 4/21/2026

Mode: Standard