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I think you're right about the brown spot being the mantle tissue. The bead at the center is made from a mollusk shell, which is also nacre. If the center bead was another pearl, it would itself...

Still not quite in focus enough to blow up the photo. But it looks like the nacre is very thick.

It's not common, but sure is interesting to be able to see perfectly what's inside the pearl. Can you take a clearer photo of the insides? I'm fascinated.

Don't throw it away. It's a great...