Basalt

Hardness: 6 on the Mohs scale, Color: Dark gray to black, Luster: Dull to sub-vitreous, Crystal structure: Aphanitic (fine-grained), Cleavage: None, Specific gravity: 2.8 - 3.0

Formed from the rapid cooling of low-viscosity lava rich in magnesium and iron exposed at or very near the surface of a planet or moon. Geological age ranges from Precambrian to recent volcanic activity.

Used in construction as crushed stone for road bases, concrete aggregate, railroad ballast, and in the production of rock wool insulation.

Basalt is the most common rock type in the Earth's crust underlying the ocean basins and is also found abundantly on the Moon, Mars, and Venus.

Identify in the field by its dark color, heavy weight, and very fine-grained texture where individual crystals are typically invisible to the naked eye. Found globally in volcanic provinces and oceanic crust.