Schist is metamorphic rock with well developed foliation. It often contains significant amounts of mica which allow the rock to split into thin pieces. It is a rock of intermediate metamorphic grade between phyllite and gneiss. The specimen shown above is a "chlorite schist" because it contains a significant amount of chlorite. It is about two inches (five centimeters) across.
Lewisian Gneiss Rock, Isle of Lewis -the Geology of the Outer Hebrides is important - as the predominant rock type is a Lewisian Gneiss - a metamorphic rock which is astonishingly up to 3 billion years old, making it the oldest rock in Britain - two thirds the age of the Earth.