Basalt

Hardness 5-6 on Mohs scale, dark black to gray color, dull to sub-aphanitic luster, fine-grained aphanitic crystal structure, specific gravity around 2.8-3.0.

Formed from the rapid cooling of low-viscosity basaltic lava exposed at or very near the Earth's surface, common in volcanic regions and oceanic crusts.

Extensively used in construction as crushed aggregate for roads, railroad ballast, building foundations, and in manufacturing stone wool insulation.

Basalt is the most common rock type in the Earth's crust underlying the world's ocean basins and also forms massive geological formations like the Columbia River Flood Basalts and the Giant's Causeway.

Identified by its dense, dark, fine-grained appearance, often with small visible vesicles or phenocrysts, found globally in volcanic provinces and island arcs.