Basalt

Hardness: 6 (Mohs), Color: Dark gray to black, Luster: Dull to sub-metallic, Crystal Structure: Aphanitic (fine-grained), Specific Gravity: 2.8-3.0

Formed via the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. Common in oceanic crust and volcanic regions.

Extensively used in construction for road base, concrete aggregate, and railroad ballast; some varieties are used for stone pavers or in rock wool insulation.

Basalt is the most common rock on Earth's surface and also covers large areas of the Moon, Mars, and Venus in the form of lunar maria and volcanic plains.

Identify by its weight (dense), dark color, and lack of visible crystals without a lens. Common in riverbeds and volcanic fields. Collectors often look for vesicles (gas bubbles) or olivine inclusions.