Basalt

Physical Properties

Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull/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 exposed at or very near the surface of a terrestrial planet or a moon. Most common rock on Earth's surface, particularly in oceanic crust.

Uses & Applications

Used widely in construction as an aggregate for roads, railway ballast, and in concrete. Also used for high-quality paving stones and mineral wool insulation.

Geological Facts

Basalt makes up nearly the entire ocean floor and is also found on the Moon and Mars. When cooling under certain conditions, it can form unique hexagonal columns, such as at the Giant's Causeway.

Field Identification & Locations

Identify by its dark color, high density, and fine-grained texture which lacks visible crystals. Commonly found in volcanic regions, oceanic islands (like Hawaii), and mid-ocean ridges.

Identified on: 5/3/2026

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