Basalt

Hardness: 6 (Mohs), Color: Dark gray to black, Luster: Dull/Matte, Structure: Fine-grained aphanitic, Specific Gravity: 2.8-3.0

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the surface of a terrestrial planet or moon. Occurs in all geological ages but is most common in oceanic crust.

Used primarily in construction for road base, concrete aggregate, railroad ballast, and floor tiles. High-quality varieties are used for stone sculpture.

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

Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture. Found worldwide in volcanic regions and ocean basins.