From Longman Dictionary of Contemporary Englishrock saltˈrock salt noun [uncountable] HEGa type of salt which is obtained from under the ground → sea salt
Examples from the Corpus
rock salt• There were runs on bread, milk, firewood, rock salt and snow shovels.• Improving rock salt distribution £350,000 saved by: Reducing number of suppliers from 75 to 11.• Start doing weekly water changes of 10-20%, but don't add any more rock salt.• Like eating two pounds of rock salt. 