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Pumice stone is fused volcanic glass,and it is also friable,take a screw driver and drag it across a pumice stone with pressure,it crumbles.
You get one micro chip imbedded in your strop and it will...

JMO guys,but I would not use a pumice stone on a leather strop.just my two cents.

Just a suggestion.If you have them,use your DMT plates for the sanding,they work great and leave no chance of possible imbedded sandpaper grit in the leather.