Basalt

Hardness: 6-7 (Mohs); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.8-3.0; Cleavage: None.

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.

Commonly used in construction as aggregate (crushed stone) for road base, railroad ballast, and concrete; also used in stone carvings and as thermal insulation in the form of rock wool.

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 in the field by its dark color, heavy weight (high density), and fine-grained texture. Often found in volcanic regions, oceanic islands, and mid-ocean ridges. Collectors look for vesicular basalt (with gas bubbles) or basalt containing olivine crystals.