Igneous rocks form through the cooling and solidification of magma or lava. Igneous rocks may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. The set includes Andesite, Basalt, Diorite, Dolerite, Gabbro, Granite Black, Granite Porphyry, Granite Red, Granite White, Kimberlite, Obsidian Black, Obsidian Green, Pumice, And Rhyolite. Average sizes are 2 cm X 3 cm X 1.5 cm