Basalt (Weathered)

Physical Properties

Hardness (Mohs): 6-7. Color: Grey-black to dark green, weathering to brown/tan. Luster: Dull or earthy. Structure: Fine-grained (aphanitic), sometimes vesicular or containing micro-phenocrysts.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. Basalt is common throughout Earth's history, covering much of the ocean floor and large continental provinces.

Uses & Applications

Extensively used in construction for aggregate, road base, and railroad ballast. Fine-grained basalt is also used for building stone, floor tiles, and sometimes in stone wool insulation.

Geological Facts

Basalt is the most common volcanic rock on Earth and also makes up the vast majority of the lunar mare (the dark plains) on the Moon.

Field Identification & Locations

Identify by its heavy weight (high specific gravity), dark interior when broken, and fine-grained texture. Commonly found in volcanic regions, oceanic islands, and mid-ocean ridges.

Identified on: 5/2/2026

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