Basalt

Hardness: 6-7 (Mohs scale); Color: dark gray to black; Luster: dull to earthy; Crystal structure: aphanitic (fine-grained); Specific gravity: 2.8–3.0.

Forms from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or moon. It is the most common volcanic rock on Earth, making up most of the ocean floor.

Extensively used in construction as aggregate for roads, railroad ballast, and in concrete. It is also used as dimension stone for tiles and monuments, or as thermal insulation (rock wool).

Basalt is not only common on Earth but also makes up large parts of the Moon's surface (the lunar maria) and the surface of Mars and Venus. It is the primary rock type found in the oceanic crust.

Identify by its heavy weight (high density), dark color, and fine-grained texture with tiny visible crystals. In the field, it often appears in hexagonal columns if it cooled slowly in a thick flow. Common in volcanic regions like Hawaii or Iceland.