Basalt

Hardness of 6 on the Mohs scale; dark grey to black color; fine-grained (aphanitic) crystal structure; dull to sub-vitreous luster; specific gravity around 2.8 - 3.0

Formed from the rapid cooling of low-viscosity, iron- and magnesium-rich lava exposed at or very near the surface of a planet or moon. Geological age ranges from Precambrian to recent volcanic activity.

Used in construction as crushed stone for road bases, concrete aggregate, and railroad ballast, as well as in manufacturing stone wool insulation.

Basalt is the most common rock type in the Earth's crust underlying the ocean basins, and it is also the primary volcanic rock found on the Moon, Mars, and Venus.

Can be identified in the field by its dark color, heavy weight relative to its size, and extremely fine grain size where individual crystals are not visible to the naked eye. Found globally in volcanic regions and oceanic crusts.