Basalt

Hardness of 6 on the Mohs scale, dark gray to black in color, fine-grained to aphanitic texture, dull to sub-vitreous luster, specific gravity around 2.8-3.0.

Formed from the rapid cooling of low-viscosity lava rich in iron and magnesium exposed at or very near the Earth's surface, typically from volcanic eruptions.

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

Basalt is the most common rock type in the Earth's crust, underlying most ocean basins and forming massive geological features like the Columbia River Basalt Group and the Giant's Causeway in Northern Ireland.

Can be identified in the field by its dark color, heavy feel relative to its size, and fine-grained texture where individual crystals are often too small to see without a microscope. Commonly found in volcanic regions worldwide.