Basalt

Hardness: 6 (Mohs scale), Color: Dark grey to black, Luster: Dull to sub-metallic, Crystal Structure: Aphanitic (fine-grained), 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. This specimen shows a fine-grained texture indicative of extrusive flow.

Extensively used in construction for aggregate, road base, railroad ballast, and high-quality concrete. Also used for stone carving 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 and Mars. Dark 'seas' on the Moon (Maria) are actually giant basaltic plains funded by ancient volcanic eruptions.

Identify by its weight (dense), dark color, and fine crystalline surface that sparkles slightly under direct light. Often found in volcanic regions, riverbeds downstream from volcanic ranges, and oceanic islands.