sandstone vs greensand

sandstone and greensand both are nouns.

Word	Noun	Adjective	Verb	Adverb
sandstone	Yes	No	No	No
greensand	Yes	No	No	No

As nouns, greensand is a hyponym of sandstone; that is, greensand is a word with a more specific, narrower meaning than sandstone:

sandstone: a sedimentary rock consisting of sand consolidated with some cement (clay or quartz etc.)
greensand: an olive-green sandstone containing glauconite

Other hyponyms of sandstone include firestone, holystone, grit, gritrock, gritstone, brownstone, bluestone, siltstone.

sandstone (noun)	greensand (noun)
a sedimentary rock consisting of sand consolidated with some cement (clay or quartz etc.)	an olive-green sandstone containing glauconite