brilliancy (n.)

"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.

abstract noun suffix of quality or rank, from Latin -cia, -tia, from Greek -kia, -tia, from abstract ending -ia (see -ia) + stem ending -c- or -t-. The native correspondents are -ship, -hood.

"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.