Alums are useful for a range of industrial processes. They are soluble in water; have a sweetish taste; react acid to litmus; and crystallize in regular octahedra.
Black salt, is known as rock salt or kala namak, is specially find in central India. Their looks is finely ground, pinkish gray salt is rich in minerals. Its smoky, tangy taste heightens the flavor in foods.
Rock salt is the common name for halite. It is a rock, rather than a mineral, and this is what makes it different to the salt you might find on your table, although it does share many characteristics.