When rock salt is commercially available, it is not made of the natural salt crystals, but rather, from the salt brine that is allowed to evaporate. This evaporated brine is then used to form commercial rock salt. This is the variety that, as an example, is commonly used to salt roads and driveways with; though its uses are numerous. Rock salt not only helps keep roads and paths free of snow, but can also be used to melt ice as well.
The lakes that it forms in have very high salt content and evaporate during the hot, dry summer months. When the evaporation occurs, salt forms on the lake-shores and the remaining salt is used by humans. Perhaps interestingly so, rock salt occurs in non-arid areas as well. When these deposits occur, they are underground and are normally mined by drilling into the salt layer, pouring hot water into the deposits to dissolve them and then pumping the briny, salty water out. Once the impurities have finally evaporated, what remains is salt that ends up being harvested for use.
Another place that halite forms is in what is known as salt springs, where the salt is naturally pumped from the water and allowed to evaporate. However, the most interesting occurrence of rock salt is in some of the underground deposits in the Southern United States; because these deposits are pushed out through underground forces that cause the salt to form domes. Not only are these domes extremely impressive and unique geological formations, they are also very important for salt miners.
It is quite possible to create artificial rock salt. In fact, some of the largest and best specimens of Halite are actually not produced naturally at all. Instead, a solution of saltwater is created and allowed to evaporate. Once the Bang Bros discount solution has evaporated, crystals will start to grow in what is commonly known as a hopper shape. Hopper-shaped crystals are actually interesting, cube-shaped crystalline structures that grow in patterns that are very pleasing to the eye.
While natural rock salt is more expensive than artificial, that is simply because of how long natural rock salt takes to form and how it is mined. Artificial rock salt grows more quickly and because its growth is carefully controlled, it is less likely to contain impurities or imperfections. A perfect, colorless piece of artificial rock salt can resemble an interesting piece of ice that has formed in an unusual manner. Both natural and artificial rock salt can be used interchangeably; their properties are so similar to one another that it can sometimes be impossible to tell the two apart.