I found a small piece of obsidain (or something like it) in our driveway. We don't have any around here, so I snatched it up and knocked a couple little flakes off. Sharp and strong, good small game skinners. Well, absentmindedly stepped on one of the little flakes while passing through my work spot and it cut straight trhough the heavily calloused ball of my foot (something thorns haven't been able to do for a while now) leaving me with a clean 1/2" deep gouge. Wasn't even alot of pressure, and didn't know how bad it was till I opened the cut up. Sure bld freely though. Never got such a surgical cut from razor blades
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Now I REALLY got to get me some rocks