Continuing our ice theme...

Iridescence happens when a thin layer of one material is viewed through another material with a very different refractive index (RI = the measure of how much something bends light). The combination, like a prism, breaks white light into its constituents. Think of oil on water or soapbubbles. Here, tiny cracks in the ice are filled with air. Pity this didn't photograph better, it was beautiful. The thing at the bottom is a lemon; ne pas de rainbows.

I misspent hours of my callow youth trying to make iridescent effects in paint, but as most of my guesses involved non-drying oils, it was unwise (and failed).