rigor vs hardness

rigor and hardness both are nouns.

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

As nouns, rigor and hardness are synonyms defined as:

rigor (noun)	hardness (noun)
something hard to endure	a quality of water that contains dissolved mineral salts that prevent soap from lathering
the quality of being valid and rigorous	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