Basalt

Hardness: 6 (Mohs scale), Color: Dark grey to charcoal, Luster: Dull/Matte, Crystal structure: Aphanitic (fine-grained), Cleavage: None/Irregular, Specific gravity: 2.8 - 3.0

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. This specific specimen shows signs of mechanical weathering and rounding by water.

Used primarily in construction for road base, aggregate, and concrete. Also used for high-tensile mineral wool insulation and architectural stone.

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Large-scale cooling of basalt often forms unique hexagonal columns, such as at the Giant's Causeway.

Identify in the field by its dark color, heavy weight (relative to size), and very fine-grained texture that lacks visible crystals. Commonly found in coastal areas, riverbeds near volcanic ranges, and ocean basins.