I have experience with cowries that confirms what Bob says...live
collected cowries, purchased but with animal still inside, that are
green. Cypraea arabica among others. They have gotten a coating of
algae...perhaps they were a bit off their feed for a while and didn't
come out of their shells?  Anyway, then they got well and covered the
algae with a new and transparent layer of nacre. Bright green when
purchased. Don Pisor once years ago had a bunch of green arabicas from
the Philippines. I have seen one of the South African cowries in green
too, fuscodentata, I think.  They DO fade to gray after a while.
Sunlight no doubt, and I don't  think there's a way to bring the green
back.

Lynn Scheu
Louisville, KY
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American Conchologist
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