Surgeons used to use obsidian as scalpel blades as well, its silly sharp!

If you ever go up a volcano, they wont let you go up in normal shoes if there is any obsidian out there as it will go right through the shoe and open your foot up in a flash. They'll typically give you sturdy boots with metal plating in the bottom and even then, you simply wont believe how quickly the soles get sliced up.