Synonyms for Hardness (same or very similar meaning)
WordNet sense 1 (devoid of passion or feeling; hardheartedness):
callosity, callousness, insensibility, unfeelingnessWordNet sense 2 (the quality of being difficult to do):
ruggednessWordNet sense 3 (excessive sternness):
harshness, inclemency, rigor, rigorousness, severeness, severity, stiffness, rigour, rigourousnessFrom the ODE community, based on WordNetadd/edit