opalescent (adj.)

"mineral like quartz but without crystalline structure," 1590s, from French opalle (16c.) and directly from Late Latin opalus (Pliny), supposedly from Greek opallios, which is possibly ultimately from Sanskrit upala-s "gem, precious stone." Used in Middle English in Latin form (late 14c.). Related: Opaline.

word-forming element meaning "beginning, becoming, tending to be," from Latin -escentem (nominative -escens), ending of present participles of verbs in -escere.

"iridescence like that of an opal, a play of colors milky rather than brilliant," 1792; see opalescent + -ence. Perhaps via French opalescence.