Below is quoted from vocabulary.com.
... (The word) Iridescent came to be in 1796, when some enthusiastic word maker took the Latin word iris, which means "rainbow," and morphed it into an English word that describes anything giving off a luminous, rainbow sheen or that changes color in the light. It's a great descriptive word — if you can resist the urge to spell it with an extra "r."
Apologize for my dumbness but what the word will be with an extra "r?" And how is that "great descriptive?"