Gneissic rock forms from metamorphism. Metamorphism changes the form, mineralogy, texture, and sometimes chemical composition of rocks through heat, pressure, and chemically active fluids. Gneissic rock has a banded appearance due to foliation and does not split easily. Quartz is another metamorphic rock that forms from sandstone and is very hard with fused quartz grains. Sedimentary rocks cover about 75% of land and ocean floors, forming from particles of eroded material transported and deposited by water, wind, glaciers or mass movement.