"sparkling with light or luster," 1680s, from French brilliant "sparkling, shining" present participle of briller "to shine" (16c.), from Italian brillare "sparkle, whirl," perhaps from Vulgar Latin *berillare "to shine like a beryl," from berillus "beryl, precious stone," from Latin beryllus (see beryl).
The figurative sense of "distinguished by admirable qualities" is from 1848. Of diamonds from 1680s in reference to a flat-topped cut invented 17c. by Venetian cutter Vincenzo Peruzzi. Related: Brilliantly; brilliantness.
"quality of being brilliant," 1755, from brilliant + -ance. The figurative sense (of wit, intelligence, etc.) is from 1779. Distinguished from brilliancy in that the latter usually is applied to things measurable in degrees.
<a href="https://www.etymonline.com/word/brilliancy">Etymology of brilliancy by etymonline</a>
Harper, D. (n.d.). Etymology of brilliancy. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/brilliancy