Basalt

Hardness of 6 on the Mohs scale, typically dark gray to black in color, aphanitic (fine-grained) texture, dull to sub-metallic luster, specific gravity around 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.

Used extensively in construction (crushed stone for road base, concrete aggregate), railroad ballast, and sometimes in sculpture or stone countertops.

Basalt is the most common rock type in the Earth's crust underlying the world's oceans, and it also forms massive geological formations like the Columbia River Plateau and the Giant's Causeway in Northern Ireland.

Identified by its fine-grained dark appearance, often vesicular (containing gas cavities), and heavy weight. Found abundantly worldwide in volcanic regions and oceanic crust.