Rhombic Porphyry (collected from the Norfolk coast, Norfolk, UK)
Lathes and diamond shaped phenocrysts of feldspar in Tachy-andesite. This rock only originates in rift zones and has come from Norway. It is sometimes found as pebbles on Norfolk beaches. No one knows exactly how the pebbles end up on these beaches, but they were likely to have been transported to the UK by glaciation during the ice age.