Rock salt and rock gypsum are common examples of a group of chemical sedimentary rocks called evaporites.

Here's why:

* Evaporites are formed by the precipitation of minerals from water as the water evaporates.

* Rock salt (halite) is formed when seawater evaporates, leaving behind high concentrations of sodium chloride.

* Rock gypsum is formed when seawater evaporates, leaving behind high concentrations of calcium sulfate.

So, both rock salt and rock gypsum are formed by the same process of evaporation and mineral precipitation, making them members of the evaporite group.