How do you describe a pearl? It doesn’t shine like other gems. Doesn’t shimmer, has no fire. It gleams, yes. But it’s the rainbow of color that really sets it apart. That . . . you know, the pearlized effect. 😉  Its iridescence.
I can never think of a more accurate word for it, so I was a little frustrated when I looked up “iridescent” and realized it entered English just a wee bit too late for my 1780 book. As in, 16 years later, in 1796. After grumbling for a minute over not being able to justify using it (I’m nit-picky about my word choices like that), I put it aside to wonder about the word.
“Iridescent” comes from the word “iris.” Now, we all know “iris,” right? The colored part of our eyes. A flower. Yep. But apparently “iris” means rainbow in Greek–hence why the colored part of our eyes are called that. The Greeks would also use it to describe any colored circle, like the round “eyes” on a peacock feather. Pretty neat, huh?
So it makes perfect sense that we would have created a word like “iridescent” to describe that rainbow effect. If only we had created it two decades earlier . . . 😉

(Photo credited to Georg Oleschinski/Inst. f. Paläont., Uni Bonn via Wikipedia Commons)