I haven’t yet set aside an afternoon to tackle another one of these vids yet, so can’t comment on Keith’s stone. But here are sand pockets:
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A straightforward number that does exactly what it says on the tin: little holes filled with sand. In the pic above I haven't yet dug those ones out, but it's very easy, you don't need a dremel or anything, you can just poke at them a bit with a corkscrew or something and the sand''ll come out. Perhaps I'll do it in a bit.
Below is the other side of the stone, which I use. It had one sand pocket in the middle of the surface, this is what it looks like having got rid of the sand, and makes no difference now in use. Though presumably at some point stone wear might mean that new ones get revealed and need further dealing with.
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