Basalt

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Dark grey to black, sometimes greenish. Luster: Dull/Matte. Crystal structure: Microcrystalline/Aphanitic (crystals too small to see with the naked eye). Specific gravity: 2.8–3.0.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava (mafic lava) at or very near the Earth's surface. This specimen appears to be a fragment from a flow or a weathered cobble. Basalt is found across all geological eras, particularly in oceanic crust and large igneous provinces.

Uses & Applications

Extensively used in construction for crushed stone, road base, concrete aggregate, and railroad ballast. Denser varieties are used for floor tiles and monuments. It is also used as a raw material for stone wool insulation.

Geological Facts

Basalt is the most common rock type in the Earth's crust as it makes up nearly most of the ocean floor. It is also common on other planetary bodies like the Moon, Mars, and Venus.

Field Identification & Locations

Identify in the field by its high density, dark color, and fine-grained texture. It often breaks into angular fragments. In many areas, it forms striking columnar jointing (hexagonal columns). Look for it in volcanic fields or coastal regions.

Identified on: 4/26/2026

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