Basalt

Physical Properties

Hardness: 6 (Mohs scale); Color: Black to dark gray; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.8 - 3.0. Dense and heavy.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. Common in oceanic crust and volcanic regions.

Uses & Applications

Used primarily in construction (crushed stone), road base, concrete aggregate, and railroad ballast. Fine-grained varieties are used for stone sculpture and fiber manufacturing.

Geological Facts

Basalt is the most common rock type in the Earth's crust; it makes up nearly most of the ocean floor. Large surface areas on the Moon, known as Maria, are also composed of basalt.

Field Identification & Locations

Identify by its dark color, heavy weight for its size, and absence of visible large crystals. Commonly found in coastal volcanic areas and riverbeds.

Identified on: 4/25/2026

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