hard rubber vs natural rubber

hard rubber and natural rubber both are nouns.

Word	Noun	Adjective	Verb	Adverb
hard rubber	Yes	No	No	No
natural rubber	Yes	No	No	No

As nouns, natural rubber is a hypernym of hard rubber; that is, natural rubber is a word with a broader meaning than hard rubber:

hard rubber: a hard nonresilient rubber formed by vulcanizing natural rubber
natural rubber: an elastic material obtained from the latex sap of trees (especially trees of the genera Hevea and Ficus) that can be vulcanized and finished into a variety of products

Other hypernyms of hard rubber include caoutchouc, gum elastic, India rubber, rubber.

hard rubber (noun)	natural rubber (noun)
a hard nonresilient rubber formed by vulcanizing natural rubber	an elastic material obtained from the latex sap of trees (especially trees of the genera Hevea and Ficus) that can be vulcanized and finished into a variety of products