Basalt

Hardness: 6-7 (Mohs scale), Color: Dark grey to black, Luster: Dull/earthy to sub-metallic, Structure: Aphanitic (fine-grained), Specific Gravity: 2.8-3.0

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the surface of a terrestrial planet or moon. Most typically associated with oceanic crust and volcanic hotspots.

Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Also used as stone siding, floor tiles, and for high-temperature thermal insulation (rock wool).

Basalt is the most common rock type in Earth's crust and covers most of the ocean floor. It is also found in large quantities on the Moon and Mars.

Identify in the field by its dark color, heavy weight for its size, and the absence of large visible crystals. It is found globally, especially in volcanic regions like Hawaii, Iceland, and the Pacific Northwest.