hardness vs rigorousness

hardness and rigorousness both are nouns.

Word	Noun	Adjective	Verb	Adverb
hardness	Yes	No	No	No
rigorousness	Yes	No	No	No

As nouns, hardness and rigorousness are synonyms defined as:

Other synonyms of hardness include harshness, inclemency, rigor, rigour, rigourousness, severeness, severity, stiffness.

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