I always pick up any obsidian I find. Sometimes a small flake will have one sharp edge. Get to handling it and suddenly you find a way it fits between your thumb and a finger or two very comfortably, and leaves the razor edge exposed for cutting.
A few years back we were about to skin an elk and my boy asked if we could use obsidian. Why not? He got a hunk and I am not a good knapper, but I can whack a hunk with a rock until a big flake comes off. And it comes of ridiculously sharp. I sliced inside the legs and down the belly faster than any knife ever did. The sharp edge was a semi-circle about 4" long and made an incredible skinning blade and the elk was done in minutes.
And sometimes, it helps to get lucky when hunting the big stuff
A quote you might find interesting:
"Good quality obsidian fractures down to single molecules which can produce a cutting edge 500 times sharper than the sharpest steel scalpel blade ("American Medical News", Nov. 2, 1984:21)."