Basalt

Hardness: 6 (Mohs scale), Color: Dark grey to black, Luster: Dull/Matte, Crystal structure: Aphanitic (fine-grained), Cleavage: None, 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. It is the most common volcanic rock on Earth.

Widely used in construction for road base, concrete aggregate, railroad ballast, and dimension stone for pavers or countertops. Finely crushed basalt is also used as a soil amendment.

Basalt is found on almost every planet in the solar system, including Mars and the Moon (dark areas known as 'maria'). If it cools specifically slowly, it can form unique tall columns known as columnar basalt.

Identified in the field by its dark color, heavy weight (density), and fine-grained texture lacking visible crystals. It is ubiquitous in oceanic crust and volcanic island chains like Hawaii.