Basalt

Hardness: 6-7 (Mohs scale) for feldspar and quartz components. Color: Generally dark, often black or dark green, with lighter mineral inclusions (white/light grey feldspar, greenish pyroxene). Luster: Dull to sub-vitreous. Crystal structure: Typically massive, fine to medium-grained; can be porphyritic with larger crystals embedded in a finer matrix, though the image shows a fine-grained texture. Cleavage: None for the rock itself, but constituent minerals like feldspar have cleavage. Specific gravity: 2.7-3.3.

Basalt forms from the rapid cooling of basaltic lava erupted at the Earth's surface or underwater. It is primarily associated with volcanic activity at mid-ocean ridges, oceanic hot spots, and continental rift zones. Its age varies widely, from recent eruptions to billions of years old.

Basalt is widely used in construction for road aggregate (crushed stone), railroad ballast, and building blocks. It is also used in the production of basalt fiber, a material with high strength and thermal insulation properties, and as dimension stone for architectural applications. Because of its dark color, it's sometimes used in landscaping.

Basalt is the most common rock on Earth's surface and ocean floors. The 'Giant's Causeway' in Northern Ireland and the 'Devil's Tower' in Wyoming are famous examples of basaltic columnar jointing. It is also found extensively on the Moon (forming the lunar maria) and other planetary bodies.

In the field, basalt is identifiable by its dark color, fine-grained texture (sometimes with small visible crystals), and relative density. It can often show signs of vesicles (small holes formed by gas bubbles) or flow structures. It is commonly found in volcanic regions worldwide, including Hawaii, Iceland, the Pacific Northwest of the USA, and various ocean basins. For collectors, it's a common rock, but interesting specimens might show columnar jointing, unique mineral inclusions, or interesting flow patterns.