Basalt

Physical Properties

Hardness: 6.0; Color: Dark grey to black, weathering to brown; Luster: Dull to earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8 - 3.0

Formation & Geological History

Formed from the rapid cooling of low-viscosity lava rich in magnesium and iron at or very near the Earth's surface. This particular specimen appears to be water-worn, likely from a river or coastal environment.

Uses & Applications

Extensively used in construction as aggregate for road base and concrete, railroad ballast, and sometimes polished for decorative tiles or jewelry (beads).

Geological Facts

Basalt is the most common rock type in Earth's crust, making up most of the ocean floor. It is also found extensively on the Moon and Mars.

Field Identification & Locations

Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture. Check for small vesicles (pitting) or a rusty exterior from iron oxidation. Commonly found in volcanic regions and along shorelines.

Identified on: 4/16/2026

Mode: Standard