Like rickharris said, you don't have to do anything to preserve them, but if you want to protect them from damage, use a two part epoxy resin. There are many products available that you mix up and simply pour over the surface and it will create a very hard surface that is much easier to clean and protects the shells.This is one that I've used. You can get it from most hardware stores and Michaels.
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Yes you can - They need no preservative as long as they are clean.Many tropical shells are used in Jewellery - Abalone for examplehttp://en.wikipedia.org/wiki/AbaloneMother of pearl has long been used for inlay work.