Basalt

Hardness: 6 on Mohs scale; Color: black to dark gray; Luster: dull to sub-metallic; Crystal structure: aphanitic (fine-grained) to porphyritic; Cleavage: none; 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 moon. Most specimens date from the Cenozoic era, but can be found across all geological ages.

Used primarily in construction for crushed stone, aggregate, and railroad ballast. It is also used to make stone wool insulation and occasionally for decorative landscaping or sculpture.

Basalt is the most common rock type in Earth's crust and makes up most of the ocean floor. It is also very common on the Moon and Mars.

Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It often shows small holes called vesicles from gas bubbles. Common in volcanic regions like the Pacific Northwest, Hawaii, and Iceland. For collectors, look for specimens with large olivine or plagioclase phenocrysts (crystals).