Most adjectives are comparable, meaning that they have a comparative form and a superlative form. For example, happy has the comparative form happier and the superlative form happiest, while intelligent has the comparative form more intelligent and the superlative form most intelligent. But iron as an adjective has no comparative or superlative, because something cannot be "ironer" than something else. It either is iron or it is not.
A list of non-comparable adjectives can be found at Non-comparable adjectives.