Aqueous solutions of transition metals cations react with sulfide sources (HS, NaHS, NaS) to precipitate solid sulfides.  Such inorganic sulfides typically have very low solubility in water, and many are related to minerals with the same composition (see below).  One famous example is the bright yellow species CdS or "cadmium yellow".  The black tarnish formed on sterling silver is AgS.  Such species are sometimes referred to as salts.  In fact, the bonding in transition metal sulfides is highly covalent, which gives rise to their semiconductor properties, which in turn is related to the deep colors.  Several have practical applications as pigments, in solar cells, and as catalysts. The fungus Aspergillus niger plays a role in the solubilization of heavy metal sulfides.