Basalt

Hardness: 6 on the Mohs scale, Color: Dark gray to black, Luster: Dull to sub-vitreous, Crystal Structure: Aphanitic (fine-grained), dense, and cryptocrystalline, Specific Gravity: 2.8 - 3.0

Formed from the rapid cooling of low-viscosity lava rich in magnesium and iron exposed at or very near the Earth's surface, typically associated with volcanic eruptions and oceanic crust formation.

Used extensively in construction (crushed stone for road base, concrete aggregate), railroad ballast, and sometimes in manufacturing tiles, countertops, and insulation fibers.

Basalt is the most common rock type in the Earth's crust, primarily making up the ocean floors. It is also found on the Moon (forming the dark lunar maria) and Mars.

Identify by its dark gray to black color, fine-grained (aphanitic) texture where individual crystals are not visible to the naked eye, and sometimes vesicular texture due to trapped gas bubbles. Found globally in volcanic regions and ancient lava flows.