Basalt

Hardness of 6 on the Mohs scale; dark gray to black in color; dull to sub-to-glassy luster; fine-grained (aphanitic) crystal structure; specific gravity ranges from 2.8 to 3.0.

Formed from the rapid cooling of low-viscosity, magnesium- and iron-rich lava exposed at or very near the surface of a planet or moon. Can be millions to billions of years old.

Commonly used in construction, road aggregate, railroad ballast, manufacturing of stone wool insulation, and occasionally as dimension stone.

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

Identify in the field by its dark color, heavy weight relative to its size, and very fine-grained matrix where individual crystals are not visible to the naked eye. Found worldwide in volcanic provinces and oceanic ridges.