Is it a dead shell? If so, do you know it’s history? Where it came from, or who collected it?
The reason I ask is that shells, especially univalves (a one part shell, vs two parts) can have animal parts remaining, or chemicals that may have been used, etc.
In any event, I would recommend a good soak in deionized or distilled water before using it. Perhaps two overnight soaks. If you suspect any part of the animal might remain, place the shell near an ant bed for a couple days. They will remove anything edible, that’s not toxic. Then do the soak(s).