The elements in metamorphic rocks do not change due to high temperatures and pressures. Iron will remain iron, silicon will still be silicon, and so forth. The basic chemical composition of the rocks remains the same before and after metamorphism. For example, limestone is a sedimentary rock made largely of calcium carbonate; the right conditions of high temperatures and pressures, usually found in mountain-building regions, can recrystallize the minerals in limestone to make marble, which also consists mostly of calcium carbonate. Likewise, sandstone, a sedimentary rock made from sand-sized grains of quartz, can metamorphose into quartzite, a hard rock consisting of interlocking crystals of quartz.