Phonolite igneous rock. This rock consists of feldspar, nepheline, pyroxene and amphibile minerals. It is formed when molten volcanic lava solidifies. It often (as here) contains larger mineral crystals held within the fine- grained matrix, forming what is known as a porphyritic texture. The name 'phonolite' refers to the ringing sound the rock makes when struck. This sample measures 115 millimetres across.