Basalt

Physical Properties

Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to brown or rust; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; 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. This specimen shows typical terrestrial weathering patterns and likely dates from various volcanic periods ranging from Cenozoic to Precambrian.

Uses & Applications

Used extensively in construction as crushed stone for road bases, concrete aggregate, and railroad ballast. It is also used as high-temperature stone wool insulation and occasionally for decorative paving and sculpture.

Geological Facts

Basalt is the most common rock type in the Earth's crust, covering most of the ocean floor. It is also common on the Moon, Mars, and Venus, where it forms the vast lunar maria.

Field Identification & Locations

Identify by its heavy weight (density), dark uniform color, and fine-grained texture. To distinguish it in the field, look for a lack of visible quartz crystals and its tendency to break into angular blocks. Common in volcanic regions such as the Pacific Northwest, Hawaii, and Iceland.

Identified on: 5/5/2026

Mode: Standard