Iron, when used metaphorically, refers to certain traits of the metaliron. Used as an adjective and sometimes as a noun, it refers to something stern, harsh, strong, unyielding, inflexible, rigid, sturdy, strong, robust, hard.[1][2][3]
Theodore Wertime and James Muhly, eds., The Coming of the Age of Iron (Yale, 1980, ISBN 0-300-02425-8) (hardcover)
"Iron, Master of Them All" from the University of Iowa Museum of Art and Project for the Advanced Study of Art and Life in Africa, discussing various metaphors (associated with iron) in African cultures.