Basalt

Physical Properties

Hardness: 6.0; Color: Dark grey to black, weathering to brown; Luster: Dull/Earthly; 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 surface of a terrestrial planet or moon. Basalt comprises more than 90% of all volcanic rock on Earth.

Uses & Applications

Extensively used in construction for road base, concrete aggregate, railroad ballast, and dimension stone for floor tiles. High-quality specimens are used in 'basalt fiber' industry.

Geological Facts

Basalt is the primary rock of the Earth's oceanic crust and is the most common rock on the surface of the Moon and Mars. On Earth, it forms spectacular columnar formations like the Giant's Causeway.

Field Identification & Locations

Identify by its heavy weight (density), dark color, and very fine-grained texture that lacks visible crystals to the naked eye. Commonly found in volcanic regions and ocean floors.

Identified on: 4/15/2026

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