Stone Type: Basalt
Basalt is known as an extrusive igneous rock because it was erupted by volcanoes as molten lava onto the Earth’s surface. It is usually black or dark gray in colour and weathers to dark green or brown. It tends to be dense and fine-grained and is often suitable for stone-flaking. Vesicular basalt is full of holes from air bubbles and is less useful for tools. Basalt is rich in iron and is composed of ca. 45-52% silica.
Basalt weathers relatively fast because the iron-rich minerals are prone to oxidising, and basalt artefacts (as well as tools made from metamorphosed volcanic and sedimentary rocks) can develop a weathered outer surface that appears pale compared to the unweathered inside. The knapping quality of basalt can vary considerably depending on the texture. Coarse-grained basalts—the most common variant—are usually very difficult to flake, but fine-grained basalts can rival the best cherts and flints. Basalt was prized for making stone axes in all parts of the world because the stone is soft enough to readily grind to shape, yet is less brittle than chert or flint, so the ground cutting edge is more durable.