Lava Stone

Scoria forms when magma containing abundant dissolved gas flows from a volcano or is blown out during an eruption. As the pressure drops, the gas forms bubbles; if the molten rock solidifies before the gas escapes, the bubbles remain as cavities. It builds cinder cones around volcanic vents and also forms the gas-rich upper parts of lava flows. The lava starts out black; oxidation of iron during eruption turns some scoria red.

Lava stone is recognised by its dark colour and pitted, bubble-filled surface. It differs from pumice, which is almost always white to light grey or tan, has so many thin-walled bubbles that it floats on water, and looks glassy under a hand lens; scoria has thicker walls, sinks in water, and often shows tiny mineral crystals. A manufactured look-alike, light expanded clay aggregate, is made by heating clay in a kiln and also has a vesicular interior.