i've seen that done on jeans. its a good distressing technique for denim.
pumice is a light rock so i don't foresee it tearing the leather (depending on the quantity and size of the pumice). the trick is to find a way to isolate the lining from the distressing elements so that it doesn't get dusty/dirty or torn up. the downside is that you can't control the distressing as much as doing it by hand.
I'd be very worried about both tears -- even small ones -- in the leather and to the damage to the stitching. I also have so idea how you could successfully protect the lining.
Someone here had a "safer" idea a while back. He took his jacket somewhere -- maybe a park -- where there was a flower bed full of rounded stones, sorta like river rock, I guess, made sure he had nothing in the pockets of the jacket, then laid it down on the rocks and stood or walked on it.
Indiana G wrote:you know,.....i could actually feel michaelson cringe in tennessee right now.....ha ha!
the force is with me....how do you think i'm typing this with a straight jacket on!?!?!
At least you don't have to worry about digging out handfuls of pumice sand whenever you put your hands in your pockets... I remember "sandwashed" jeans!