Subject: Could a fossilized unopened (oyster) shell still contain a pearl?

I found some type of shell which is hard as a rock and looks fossilized. 
It got me to thinking about the oyster and pearl question. So I guess part 
of my question is how durable a pearl is in the environment - would it's 
original makeup be changed by some type of replacement process over time?
I'm thinking something like a diamond formed under great pressure and non-
organic would last almost forever (maybe I've just seen too many DeBeers 
commercials). How about amber - this is an organic substance but has 
lasted looking like hardened tree sap for millions of years, though 
perhaps some of it's chemistry has changed?

Re: Could a fossilized unopened (oyster) shell still contain a pearl?