Basalt

Physical Properties

Hardness: 6 on Mohs scale; Color: Dark grey to black, occasionally greenish-black; Luster: Dull to earthy; Crystal Structure: Aphanitic (fine-grained) to 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 very near the Earth's surface. It is the most common rock type in the Earth's crust and makes up most of the ocean floor, typically forming in divergent plate boundaries or oceanic hotspots.

Uses & Applications

Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. It is also used as dimension stone for tiles and monuments and in the manufacturing of stone wool (insulation material).

Geological Facts

Basalt is the most common rock on the surfaces of other terrestrial planets in our solar system, such as Mars and Venus. Large areas of the Moon's surface, known as 'Maria,' are composed of ancient basaltic lava flows.

Field Identification & Locations

Identify in the field by its high density, dark color, and fine-grained texture that lacks visible crystals to the naked eye. It can often be found in volcanic regions, such as the Pacific Northwest of the US, Iceland, or Hawaii. Collectors look for vesicles (gas bubbles) that may be filled with secondary minerals like agates or zeolites.

Identified on: 4/22/2026

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