Basalt

Hardness of 6 on the Mohs scale, dark gray to black in color, dull to sub-with luster, aphanitic (fine-grained) crystal structure, 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. Geologically very common.

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

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

Easily identified by its dark, fine-grained, sometimes vesicular (bubbly) texture. Found globally in volcanic regions, ocean floors, and volcanic islands like Hawaii.