Tom's suggestion about conchiolin is a good one, I think. Some shells
contain a large percentage of this natural plastic, and it would be most
unwise to try bleach on these. Pinctada, pinnids...

Some gastropod shells are composed almost entirely of periostracum; in the
large NZ landsnails Paryphanta & Powelliphanta there is only a thin smear
of carbonate on the inside of the thick periostracum. When the shells are
allowed to dry-out the shell develops dimples, separates along the sutures
and curls up into a twisted mess.

Such shells are perhaps best treated under vacuum with paraffin oil...

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