sentences: metamorphic is a class of rock
docs: gneiss is metamorphic_rock. slate is metamorphic_rock. marble is metamorphic_rock. amphibolite is metamorphic_rock. metamorphic_rock is rock. marlite is metamorphic_rock. metamorphic_rock is stone. hornfels is metamorphic_rock. marble is useful_metamorphic_rock. marble is beautiful_metamorphic_rock.