rock is formed by the gradual depositing (or sedimentation) of tiny pieces of rock, clay, and other materials. Given enough time and pressure, these substances "fuse" together to form a solid mass. Igneous
rock is formed from the hardening of lava and metamorphic
rock is formed through changes in temperature or pressure, or by chemical means. Gneiss
is not a rock classification category. It is a rock that is classified as metamorphic.