hardness

(noun) the property of being rigid and resistant to pressure; not easily scratched; measured on Mohs scale.

(noun) a quality of water that contains dissolved mineral salts that prevent soap from lathering. Example: "The costs of reducing hardness depend on the relative amounts of calcium and magnesium compounds that are present."

(noun) devoid of passion or feeling; hardheartedness. Synonyms: unfeelingness, callousness, callosity, insensibility.

(noun) the quality of being difficult to do. Synonyms: ruggedness. Examples: "He assigned a series of problems of increasing hardness." "The ruggedness of his exams caused half the class to fail."

(noun) excessive sternness. Synonyms: severity, severeness, harshness, rigor, rigour, rigorousness, rigourousness, inclemency, stiffness. Examples: "Severity of character." "The harshness of his punishment was inhuman." "The rigors of boot camp."