Has anyone heard of this one?

I was cleaning it up and noticed that it was deep into the wood, so I worked on getting it out. Stone is 9 1/4 long x 1 5/8 wide x1 5/8 thick. Stone seems to be a very fine stone & maybe a finisher. It is definitely a natural. What have I got here?

It came from the United Kingdom if that helps...

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Thank you in advance!