firmness the property of being unyielding to the touch
hardness -
the quality of being difficult to do; "he assigned a series of problems of increasing hardness"; "the ruggedness of his exams caused half the class to fail"
difficultness,
difficulty the quality of being difficult; "they agreed about the difficulty of the climb"
hardness -
a quality of water that contains dissolved mineral salts that prevent soap from lathering; "the costs of reducing hardness depend on the relative amounts of calcium and magnesium compounds that are present"
hardness -
excessive sternness; "severity of character"; "the harshness of his punishment was inhuman"; "the rigors of boot camp"