Andesite. Cut surface of amygdaloidal andesite, a type of volcanic rock formed from solidified lava. Gas bubbles in the lava (called amygdales) in this sample have filled with crystalline material, creating coloured patches. Andesite is a fine- grained volcanic rock consisting of the mineral plagioclase feldspar with biotite, hornblende or pyroxene. Magnification x0.5 at 35mm size.