brilliant vs brilliancy

brilliant is not a noun while brilliancy is a noun.

Word	Noun	Adjective	Verb	Adverb
brilliant	No	Yes	No	No
brilliancy	Yes	No	No	No

brilliant (noun)	brilliancy (noun)
	a quality that outshines the usual

brilliant (adjective)	brilliancy (adjective)
full of light; shining intensely
having strong or striking color
characterized by grandeur
having or marked by unusual and impressive intelligence
clear and sharp and ringing
of surpassing excellence