I read somewhere that pearls have a kind of “iridescence,” because of the way they are made. How does a pearl look under ultraviolet light?
Two sentences and somehow they seem a nonsequitur… Are you confusing “iridiscence” with “phosphorescence” or something like that? Iridiscent means “akin to the rainbow” (called “the bow of Iris” in Greek mythology and in many languages influenced by that); pearls and nacre have a multicolored glow.
The iridescence of pearls is similar to the rainbow effect you get from a thin layer of oil on top of water: Both are due to having layers of a thickness comparable to the wavelength of light, so you get constructive and destructive interference of various colors. The same effect probably continues into the near UV range, even though you can’t see it. Get too far out, though, and the wavelength will be significantly shorter than the layer thickness, and the effect would end.
Pearls fluoresce under longwave UV light, and the colour is somewhat dependant on where it’s grown. Apparently it’s one way to check for natural pearls. http://www.pearl-guide.com/forum/showthread.php?5354-Natural-Pearl-(diamond-amp-platinum)-earclips-late-Art-Deco