Rock salt is salt that forms in large crystals or chunks underground, rather than the fine grains you sprinkle on food. It looks like small rocks or pebbles, sometimes clear like ice, sometimes white or grayish. When miners dig it out of ancient salt deposits, they find huge slabs that get crushed down for different uses.

You see rock salt most often scattered on icy roads and sidewalks in winter. When salt touches ice, it lowers the freezing point of water, making the ice melt even when the temperature stays below 32°F. Cities buy rock salt by the truckload and spread it with special vehicles. This salt is too coarse and impure to eat, but it does the job of keeping streets safe.

People also use rock salt in old-fashioned ice cream makers, where it surrounds the ice cream container and helps freeze the mixture inside. Some cooks use food-grade rock salt for special recipes or in salt grinders. The word halite is the scientific name for rock salt, the mineral form of sodium chloride. Underground, halite deposits formed millions of years ago when ancient oceans dried up, leaving behind thick layers of crystallized salt.