Basalt

Hardness of 6 on the Mohs scale, dark gray to black in color, fine-grained (aphanitic) crystal structure, dull to sub-vitreous luster, specific gravity around 2.8-3.0.

Formed from the rapid cooling of low-viscosity, iron- and magnesium-rich lava exposed at or very near the surface of a planet or moon, often associated with volcanic eruptions and fissure vents.

Commonly used in construction as crushed stone for road bases, concrete aggregate, and railroad ballast, as well as in the production of basalt fiber.

Basalt is the most common rock type in the Earth's crust underlying the oceans, and it also forms massive volcanic features on other terrestrial bodies like the Moon and Mars.

Easily identified by its dark, fine-grained appearance, often heavy feel, and occasional vesicular texture (gas bubbles); found worldwide in volcanic regions, oceanic islands, and large igneous provinces.