Basalt

Physical Properties

Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull/Sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Density: 2.8–3.0 g/cm³.

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. Most basalt on Earth is formed at mid-ocean ridges through seafloor spreading.

Uses & Applications

Commonly used in construction for aggregate, road base, railroad ballast, and ground-stone pavers. High-quality varieties are used in 'basalt fiber' production for industrial materials.

Geological Facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also extremely common on the Moon and Mars.

Field Identification & Locations

Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture where individual crystals are too small to see with the naked eye. Look for 'vesicles' (small gas bubbles) in some specimens.

Identified on: 4/14/2026

Mode: Standard