One of the reasons surgeons’ scalpels are made with obsidian is their sharpness. They are 500 times sharper than steel. Even though some say that the Aztecs inflicted much pain on their victims when cutting them with their obsidian blades, they actually inflicted much less pain with a very quick and exacting motion of this more than razor-sharp blade, involving no sawing effect, whatsoever, that would have been much more agonizing with a steel blade. Obsidian is good for weapon-making also because it is far harder than steel. The one thing people may argue that makes it not good for weapon-making is its brittleness, but as long as it’s not being used as a weapon against something harder than itself, that’s not a factor.