Basalt

Physical Properties

Hardness: 6.0 on Mohs scale; Color: Dark grey to black, sometimes greenish; Luster: Dull/Earthy; Structure: Fine-grained (aphanitic), often vesicular (holy); Specific Gravity: 2.8 - 3.0

Formation & Geological History

Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. This specimen shows a vesicular texture from trapped gas bubbles.

Uses & Applications

Used extensively in construction as an aggregate (crushed stone) for roads, railroad ballast, and concrete. Also used for high-temperature insulation materials like rock wool.

Geological Facts

Basalt is the most common rock on Earth's surface and actually makes up the entirety of the ocean floor. Most of the dark plains on the Moon (the Lunar Maria) are also composed of basaltic lava flows.

Field Identification & Locations

Look for a heavy, dark, fine-grained rock. It often feels denser than other rocks of its size. Common in volcanic regions such as the Pacific Northwest, Hawaii, and Iceland. Testing for lack of reaction with acid helps distinguish it from limestone.

Identified on: 4/24/2026

Mode: Standard