Basalt

Hardness: 6.0; Color: Dark grey to black; Luster: Dull to submetallic; Crystal structure: Aphanitic (fine-grained); 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. Most basalt flows formed during the Cenozoic era, though it has been produced throughout Earth's history.

Extensively used in construction for aggregate, road base, railroad ballast, and high-quality asphalt. Also used in manufactured stone products and as a raw material for stone wool insulation.

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found on the Moon, Mars, and Venus.

Identify by its dark color, heavy weight (relative to size), and very fine grains that are usually too small to see with the naked eye. Found commonly at volcanic sites and oceanic ridges.