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What is the provenance of the ring?
Given that it feels gritty, see if you can borrow a 10x loupe (basic equipment which I think we should all own) and look at the surface of the pearl under magnification. Real nacre is smooth-looking (apart from blemishes in the surface of the pearl) while imitation pearl coating is a bit coarser looking. If you are still on the fence, compare what you see with how a known-real and a known-fake pearl look under the loupe.