Sea salt contains 98% sodium chloride and rock salt contains a higher amount of sodium chloride as compared to sea salt. The cheap rock salt found in markets comes from underground salt mines and it may be found as black salt (Kala Namak) or Himalayan salt.
Since the rock salt is collected from mines found in different places – the composition varies from one mine to others. The healthiest forms of sea salt are least refined and it does not contain any preservative. Most refined ones are harvest and recrystallised to get crystal structure.
It may not contain additives but it may contain additional chemicals to enhance the feature of the salt.