For a long time I've known that obsidian is very sharp and that it was historically used to make stuff like arrowheads. But it wasn't until a few years ago that I learned how sharp it was. A redditor said it's used as scalpels. I thought that was copypasta nonsense but I googled anyway and I learned I was wrong.

I tried knapping obsidian and it did not go well, probably didn't help I was using a nail pounded into a wood handle. Glass is a much easier material to work with.