hard rubber vs gum elastic

hard rubber and gum elastic both are nouns.

Word	Noun	Adjective	Verb	Adverb
hard rubber	Yes	No	No	No
gum elastic	Yes	No	No	No

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

hard rubber: a hard nonresilient rubber formed by vulcanizing natural rubber
gum elastic: 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, India rubber, natural rubber, rubber.

hard rubber (noun)	gum elastic (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