Basalt is an igneous rock. It is a general term for a fine grained, dark coloured rock that has many forms. A type called traprock has been used in making roads. Basalt forms column structures at Fingal's Cave on the island of Staffa, off the coast of Scotland, and the Giant's Causeway on the coast of Northern Ireland.

Liquid basalt from the earth's crust often reaches the surface as molten lava. When this cools it forms obsidian, which is hard and brittle. As it had sharp edges it was used by early man to make axes, knives and other sharp tools.