Stone Type: Igneous
‘Igneous’ is a catch-all term to describe rocks formed when molten magma cools and solidifies. Intrusive igneous rocks form below the Earth’s surface, usually resulting in relatively coarse-grained stones like granite or diorite. Extrusive igneous rocks form when magma reaches the surface and cools and solidifies rapidly. These are also called ‘volcanic’ rocks. Volcanic rocks are often finer-grained and suitable for flaking, such as basalt, rhyolite, or obsidian.
Basalt
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.
Obsidian
An iridescent sheen can occur in obsidian caused by light reflecting from minute inclusions of minerals, debris, or gas. Most obsidian is black but it can occur in various shades of brown, tan, green, red, blue, or yellow, depending on trace elements. Obsidian is an excellent material for flaking, although it is more fragile than chert. The edge of an unmodified obsidian flake is sharper than a modern steel surgical scalpel.
