Basalt

Hardness: 6 (Mohs), Color: Charcoal grey to black, Luster: Dull/Matte, 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 moon. Most basalt flows are from the Cenozoic era, though they can date back billions of years.

Used in construction as crushed stone for road base, railroad ballast, and concrete aggregate. High-density varieties are used for coastal protection (riprap).

Basalt is the most common rock type in Earth's crust and makes up most of the ocean floor. It is also common on the Moon, Mars, and Venus, forming the dark 'seas' or maria seen from Earth.

Identify by its dark color, heavy weight (relative to size), and fine-grained texture that lacks visible crystals to the naked eye. Commonly found in volcanic regions, oceanic islands, and mid-ocean ridges.