The Isles of Lewis and Harris are formed of Lewisian gneiss. Gneiss is pronounced 'nice', and 'Lewisian' relates to the Isle of Lewis. Lewisian gneiss is a metamorphic rock formed around 3 billion years ago. The formation process may produce characteristic layers or bands of different colours within the rock, as can be seen in these gneiss boulders on a beach. The layering phenomenon is known as foliation.
http://en.wikipedia.org/wiki/Foliation_(geology)
http://en.wikipedia.org/wiki/Lewisian_complex