Devonian slates, Close-up of banded Devonian coastal rocks and coloured slates at Polzeath, Cornwall, UK, Slate is a metamorphic rock, Most of the rocks we see at the Earth's surface are sediments, sand and coloured muds which settle to the sea bed, These can be altered later as the heat and pressure of tectonic forces cause structural changes in the rocks (metamorphism), The slates vary in colour from black to yellow to brown to green to purple, Other formations and patterns arise when granite and quartz veins (white) penetrate surroundings rocks,