red coral vs precious coral

red coral and precious coral both are nouns.

Word	Noun	Adjective	Verb	Adverb
red coral	Yes	No	No	No
precious coral	Yes	No	No	No

As nouns, red coral and precious coral are synonyms defined as:

red coral and precious coral: the hard stony skeleton of a Mediterranean coral that has a delicate red or pink color and is used for jewelry

Other synonyms of red coral include coral.

red coral (noun)	precious coral (noun)
corals of especially the Mediterranean having pink or red color used for ornaments and jewelry	the hard stony skeleton of a Mediterranean coral that has a delicate red or pink color and is used for jewelry
the hard stony skeleton of a Mediterranean coral that has a delicate red or pink color and is used for jewelry