Porphyry is a type of igneous rock that is formed from volcanic magma. It has a distinctive texture, with large crystals embedded in a fine grained silicaceous matrix. The large crystals are called phenocrysts and are usually made up of minerals such as feldspar, quartz, or mica.
Porphyry is often used as a decorative stone for construction, and is the material used to create numerous famous monuments and decorative artworks since antiquity, being mentioned in Pliny the Elders ‘Natural History’.
Around the UK, rhombic porphyry can be found on beaches, and is a favourite of beachcombers.