Calamine is zinc carbonate, colored with ferric oxide (rust). Zinc carbonate is a counter-irritant used to reduce itching. It is still an irritant when applied to skin so it should be used only as needed.
Calamine contains about 5% zinc oxide (which can clog pores) and a good deal of phenol, which is extremely toxic to skin. In 1992, the FDA actually tried to ban calamine altogether, but that didn’t happen. Instead, it is classified as a skin protectant (due to its relation to zinc oxide) with usage restrictions (Source: Code of Federal Regulations, Title 21, volume 5, April 2006).