What is the difference between rock and sea salt?

Rock salt is mined from salt deposits underground, which are the remnants of prehistoric seas. Sea salt comes from collecting seawater and evaporating the water, which allows the salt to crystallize. So, in practical terms, there's not much difference between them. Both are mainly sodium chloride, with small amounts of some other salts mixed in because it's not worth the effort required to separate them out. Iodide salts are commonly added to salt that is sold for human consumption, because their inclusion in the diet prevents thyroid problems.  more