Iron sulfide

Iron sulfide is the chemical compound FeS, a black solid. It is made of iron and sulfide ions. FeS has iron in its +2 oxidation state. It reacts with acids such as hydrochloric acid to make hydrogen sulfide gas.
In plural, "iron sulfides" may refer to a range of chemical compounds composed of iron and sulfur. For example to FeS2 (iron disulfide) found in the ground as the mineral pyrite. FeS2 is shiny like a metal and has a bright golden color. That's why it is sometimes called "fool's gold". 
Natural minerals.
By increasing order of stability: