sapphire vs sky-blue

sapphire and sky-blue both are nouns.

Word	Noun	Adjective	Verb	Adverb
sapphire	Yes	Yes	No	No
sky-blue	Yes	Yes	No	No

As nouns, sapphire and sky-blue are synonyms defined as:

Other synonyms of sapphire include azure, cerulean, lazuline.

sapphire (noun)	sky-blue (noun)
a light shade of blue	a light shade of blue
a transparent piece of sapphire that has been cut and polished and is valued as a precious gem
a precious transparent stone of rich blue corundum valued as a gemstone

sapphire (adjective)	sky-blue (adjective)
of something having the color of a blue sapphire	of a deep somewhat purplish blue color similar to that of a clear October sky