rock elm vs white elm

rock elm and white elm both are nouns.

Word	Noun	Adjective	Verb	Adverb
rock elm	Yes	No	No	No
white elm	Yes	No	No	No

As nouns, rock elm and white elm are synonyms defined as:

rock elm and white elm: large ornamental tree with graceful gradually spreading branches common in eastern North America

Other synonyms of rock elm include American elm, Ulmus americana, water elm.

rock elm (noun)	white elm (noun)
tall widely distributed elm of eastern North America	large ornamental tree with graceful gradually spreading branches common in eastern North America
large ornamental tree with graceful gradually spreading branches common in eastern North America