Basalt

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

Formed from the rapid cooling of low-viscosity, mafic lava exposed at or very near the Earth's surface, typically associated with volcanic eruptions, oceanic crust creation, and mantle plumes.

Used heavily in construction (as crushed stone for road bases, concrete aggregate, and railroad ballast), manufacturing of rock wool insulation, and occasionally as decorative stone or in monuments.

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

Can be identified in the field by its dark gray to black color, fine-grained texture (individual crystals are usually not visible without a microscope), and heavy feel. It is ubiquitous in volcanic regions worldwide.