It's not, though. Please see below for extract from the Safety Data Sheet for rock salt:Not rock salt googled it before lethal stuff .......
Product Description Rock Salt is approximately 94% pure salt and has a characteristic reddish-brown colour owing to the presence of marl (an insoluble mineral) which is the chief impurity. Alternative Names: Sodium Chloride, Common Salt, Halite CAS Number: 007647 14 5 EINECS Number: 231 598 3 HAZARDOUS INGREDIENT(S) Contains no Hazardous Ingredients EC Directive 93/112/EEC ____________________________________________