ice
Apparently my old friend thin-film interference1 can operate on thin sheets of water trapped inside cracks in thick ice. Neat! I have learned a thing.
In related news, we are suffering a second freeze hard enough to block the well, the livestock are miserable, and we had to break a hole in the ice on the duckpond so some newts who thought it was spring last week and got into the pond to have dirty newt sex and lay eggs all over everything could breathe. :( I want a shower like a black hole wants a passing alpha particle.
Bleh.
(1) Thin film interference is the rainbow colors visible on very thin, transparent objects like soap bubbles, oxide layers on knives, and oil slicks. It is caused by lightwaves reflecting from the front of the object and the back of the object interfering with each other.