The two most prevalent iron oxides, black and red iron oxide, are the 16 forms that can exist naturally. They build on geologically old parent materials in humid, tropical areas with naturally occurring rainforest flora.  Others, like black iron oxide, occur in sedimentary, metamorphic, mafic, and ultramafic rocks.  Since hematite is a component of rust that develops on iron objects that come into contact with air, red iron oxide can be found in nature.