Yes, HARDNESSES is a Scrabble word!

Letter values in Scrabble US: H 4, A 1, R 1, D 2, N 1, E 1, S 1, S 1, E 1, S 1.

Is it a valid word?

HARDNESSES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the property of being rigid and resistant to pressure; not easily scratched; measured on Mohs scale
a quality of water that contains dissolved mineral salts that prevent soap from lathering
devoid of passion or feeling; hardheartedness
the quality of being difficult to do
excessive sternness

Not playable in Scrabble US: RIGOUROUSNESS.