Rainbows
The little 'rainbows' often seen on stratus cloud tops which contain your own aircraft shadow are not as rare as the full sized rain drop rainbows under raining cloud which apparently need special met conditions and sunlight to materialise.
In a lifetime of flying I have only seen the phenomenon twice and look as hard as I could I was unable to see any pot of gold.
There was a thread going on the subject a year or so ago and some fascinating photos appeared. But I have yet to see a good photo of the full circle of the full whammy
Once seen - never forgotten.