Pearls of Wisdom
Isn't that lovely? It's called a nacreous cloud. The full-sized picture's here, and there are even more pictures of them here. They're formed by clouds of ice crystals combined with dust particles and are therefore rare.
"Nacre" (from the French) means "mother-of-pearl" (the German version is the surname Perlmutter), and "nacreous" therefore means "pearly"; nacreous clouds have that opalescent rainbow effect within them, as does mother-of-pearl.
The etymology of "nacre" as offered by Answers.com is baffling: the French got it from the Italian "naccaro", which means "drum", and they in turn got it from the Arabic "naqqara", with the same meaning, and their etymology derives it from a word meaning "to bore or pierce", and frankly I'm lost.
But still, I think the word "nacre" is particularly lovely, even though it doesn't sound like much: it rhymes with "baker", for heaven's sake. (All English words that end in "-cre" are pronounced as if they end with "-ker", with one exception, "fiacre", which ends with the more or less authentically French "-cruh". It's odd that "fiacre" retained its French pronunciation while the back half of "chancre" didn't, even though its first half did. Such are the peculiarities of English pronunciation.)
Even better is "nacreous", and while you might logically think we'd pronounce it "nay-ker-us" or even "nay-kruss", given the noun's pronunciation, we don't: that "-e-" pops right back up and the whole thing becomes "nay-kree-us" (as in, say, "aqueous" or "vitreous", but not "righteous" or "curvaceous").
It seemed obvious that "nacre" should be a French word, but the surprise is that all "-cre" words aren't French. "Acre", for example, hasn't ever been anywhere near that language (it's from Old English, originally from a Nordic language, though it used to have a "-k-" instead of a "-c-"), and "lucre", which again looks French, is actually straight from Latin "lucrum". English is just full of surprises.