Basalt

Physical Properties

Hardness: 6-7 (Mohs); Color: dark gray, black, or dark brown; Luster: dull to sub-metallic; Crystal structure: fine-grained (aphanitic) or porphyritic; Cleavage: none (conchoidal fracture); 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 can be found in oceanic crust and mid-ocean ridges, as well as continental flood basalts.

Uses & Applications

Used primarily in construction for road base, concrete aggregate, railroad ballast, and building stone. Fine specimens are used as 'hot stones' in massage therapy or for decorative landscaping.

Geological Facts

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

Field Identification & Locations

Identify in the field by its high density for its size, dark color, and fine-grained texture that usually lacks visible crystals unless they are phenocrysts (like green olivine). It is ubiquitous in volcanic regions.

Identified on: 4/20/2026

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