Basalt

Hardness: 6-7 (Mohs), Color: Dark grey to black, Luster: Dull to sub-metallic, Crystal Structure: Aphanitic (fine-grained), Cleavage: None, Specific Gravity: 2.9-3.1

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. It is the most common rock in Earth's crust, primarily making up the ocean floors.

Primarily used as an aggregate in construction (roads, concrete, railroad ballast). Polished varieties are used for floor tiles and decorative stonework. It is also used in 'hot stone' massage therapy.

Basalt is also prevalent on other planetary bodies; much of the Moon's surface (the Maria) and Olympus Mons on Mars are composed of basalt.

Identify by its dark color, heavy weight (relative to size), and fine-grained texture where individual crystals are not visible to the naked eye. Often found in volcanic regions or alongside riverbeds as smooth pebbles.