The oysters grown for food are not the same type of mollusk used to grown gem pearls. You have a pearl, but it's not a gem. It's a bit like sheep. Some are raised for meat, and some are raised to provide beautiful wool. You can't get both good meat and good wool from the same breed of sheep.
Cool story on how you found it. I second getting a little cage to put it in if you'd like to wear it.