hard·ness 
/ˈhɑɚdnəs/ noun
/ˈhɑɚdnəs/ noun [noncount] : the quality or state of being hard
▪ the hardness of a diamond ▪ The wood's hardness makes it suitable for carving. ▪ There was a hardness [=toughness] in her voice I hadn't heard before.
▪ the hardness of a diamond ▪ The wood's hardness makes it suitable for carving. ▪ There was a hardness [=toughness] in her voice I hadn't heard before.
