Basalt

Basalt is the most common rock on Earth's surface. It forms the entire ocean floor and makes up massive volcanic provinces on every continent. It erupts as low-viscosity lava that flows easily, sometimes traveling tens of kilometers from its source.

Unlike granite (which cools slowly underground to form large crystals), basalt cools rapidly at the surface, producing a fine-grained texture where individual mineral grains are too small to see without magnification. The minerals present are the same as in gabbro (its slow-cooled equivalent): plagioclase feldspar, pyroxene, and olivine.

Basalt is the rock that fills the cavities where zeolites, agates, and amethyst form. When basaltic lava solidifies, trapped gas bubbles create vesicles (holes). Over millions of years, mineral-rich groundwater fills these cavities with secondary minerals, producing the geodes and crystal-lined cavities that crystal collectors prize.

Basalt is identified by its dark gray to black color, fine-grained texture (crystals too small to see), and density (noticeably heavier than granite). Vesicular basalt contains round gas-bubble holes. Columnar basalt forms dramatic hexagonal columns (Giant's Causeway, Devils Postpile).

Distinguish from obsidian (glassy, no visible texture), gabbro (same minerals but coarse-grained), and slate (layered, can split into sheets). Basalt feels dense and solid with a uniform dark appearance.