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Oops, I meant to post this pic first. Sorry.

Chris L

22652

Here you go. Does your #4 look like this stone?

22649

22650

22651

The mag shot is on only 40x with my stereo mic. This is a pretty coarse stone and somewhat gritty even when rubbing...

Very interesting group of stones. I have one identical to #4 in composition but mine is long and narrow. Volcanic is the way I'd describe it as well. I got mine with a few other sharpening stones...