Basalt

Physical Properties

Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained, 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 exposed at or very near the surface of a terrestrial planet or a moon. It is the most common volcanic rock on Earth, covering most of the ocean floor.

Uses & Applications

Extensively used in construction, specifically as road base, railroad ballast, and aggregate in concrete. Dense specimens are also used for hot stone massage and decorative landscaping.

Geological Facts

Basalt is not only widespread on Earth but is also the primary rock type of the lunar mare on the Moon and covers much of the surface of Mars and Venus. Giant's Causeway in Northern Ireland is a famous example of columnar basalt.

Field Identification & Locations

Identify by its dark color, heavy weight (relative to size), and lack of visible crystals. It can often be found in volcanic regions, oceanic islands, or as smoothly weathered river and beach stones in those areas.

Identified on: 4/23/2026

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