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 at or very near the Earth's surface. It is the most common volcanic rock on Earth and makes up most of the ocean floor and oceanic islands.

Extensively used in construction for aggregate, road base, asphalt, and railroad ballast. It is also carved into sculptures or used as paving stones.

Basalt is also found on other planetary bodies, including the Moon (lunar maria) and Mars, indicating significant volcanic history elsewhere in the solar system.

Identify by its dark color, heavy weight (density), and fine-grained texture. It often shows 'vesicles' (tiny gas bubble holes) or a 'scoriaceous' texture. Common in volcanic regions.