Basalt

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Dark grey to black; Luster: Dull/Matte; Crystal structure: Fine-grained/Aphanitic (crystals too small to see with naked eye); Specific gravity: 2.8-3.0.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava (mafic) at or very near the Earth's surface. It is the most common rock type in the Earth's crust, making up most of the ocean floor.

Uses & Applications

Majorly used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. It is also used in high-durability floor tiles and as 'basalt fiber' for industrial reinforcement.

Geological Facts

Basalt is found on other planetary bodies, including the Moon (forming the dark 'maria'), Mars, and Venus. It is the volcanic equivalent of Gabbro.

Field Identification & Locations

Identify in the field by its dark color, heavy weight (density), and fine-grained texture that lacks visible crystals. Commonly found in volcanic fields, oceanic islands (like Hawaii), and mid-ocean ridges.

Identified on: 4/17/2026

Mode: Standard