The rarest nymph is subjective I guess.. for me it's Light and Air. 0 and 0 of those on my end.
Statistically, there shouldn't be a "rarest nymph" unless there's a glitch, like that devourer bias thing.
First of all I cannot verify any of this below to be true as I don't have any swf decompiler software nor reverse engineering skills with flash.
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According to people who have used flash editing software and opened the game code, there's 50% higher chance to win an entropy nymph compared to the other nymphs.
How?
According to this info, the oracle wheel picks an element then a card, if the a nymph is rolled it will roll again and if its a nymph the second time too you will get it. With entropy nymph the extra check doesn't occur.
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Make of it what you will.