There has been a rather sharp increase (in my experience at least!) of
dead cowries being coated with a very life-like plastic coating to make
them appear live.  The plastic used has a gloss very similar to the
normal gloss of Cypraea, so the effect can be quite convincing if the
specimen is fresh-dead and the colors are not that faded yet.  Caveat Emptor......

From Rainy old New Scotland,
Ross M.