Salt is harvested from the earth in one of two ways. The salt can be evaporated from seawater or it can be mined from deposits in the ground. The latter form of salt is called rock salt, also known as halite or land salt. Rock salt crystals are larger, less refined, and more discolored than purified table salt. They often contain impurities such as gypsum and potassium chloride. However, they do not contain additives such as iodine or anti-caking agents commonly found in table salt. The salt is sold in most grocery stores or health food stores in either edible/gourmet or inedible forms. The inedible form is not dangerous, however it does contain grit that would make it unpleasant to eat.
Rock salt has many uses. Its best known use is to help make homemade ice cream in an electric or hand-crank machine. Rock salt and ice are placed in a large bucket and a container holding the chilled ice cream mixture is placed inside it. The salt lowers the freezing point of the ice, which causes it to melt. Melted ice water is much better at absorbing the heat from the ice cream mixture than ice cubes and this makes the ice cream freeze faster. Rock salt is the best salt for ice cream because of its large grains. If table salt is substituted cup-for-cup then the ice/salt mixture will contain more salt and the ice cream will freeze too fast and not be as creamy.
The edible form of rock salt can be used to bake meats and root vegetables such as potatoes. The food item is surrounded with rock salt and baked in an oven. The salt aids the cooking process and seasons the food. Sometimes the salt is moistened with water before baking to create a salt crust that retains the moisture of the food. This hard crust must be chipped away to access the food. There are various recipes online for cooking foods in rock salt.
Ouroboros adds:"Just last night I baked a trout in a rock salt crust. I used egg whites and water (1Tbs water/eggwhite) to mix up a granita of salt. 450deg for 15min. a sprig of rosemary laid inside the boned fish was perfect." yclept also thinks using egg whites instead of water to moisten the salt is a good idea, as it is less messy.
Rock salt also has many other uses besides food. The inedible form is commonly sprinkled on roads and sidewalks during winter to remove and prevent the formation of ice. The grit in the salt helps to add traction to the surfaces. The edible form of the salt is also added to aquarium tanks to keep fish healthy and used with aromatherapy oils in baths. Additionally, some sources say grass sprinkled with rock salt is more appealing to cattle and soil enriched with rock salt will help sugar beets produce more sugar.
http://switcheroo.com/Salt.html
http://www.saltinfo.com/salt%20surprises.htm
http://www.saltinstitute.org/14.html