What is rock salt?

Rock salt, also known as halite, is the mineral form of sodium chloride (NaCl). It is usually colorless or white, but it may contain a wide variety of colors depending on impurities. Chemically, it is the same as table salt. Physically, however, it is much coarser (about the consistency of gravel). In northern climates, it is used to melt ice on streets because brine (a mixture of salt and water) has a lower freezing point than pure water.