Obsidian

Obsidian is found in several different colours; it is molten lava which has cooled down very quickly and therefore not been able to crystallise. It is in affect "natural glass" and like glass usually has very sharp edges which is probably why it was used by our Ancients as a weapon when it had been shaped similar to our Black Obsidian Arrowheads. (https://holistic-crystals.co.uk/p/black-obsidian-arrowhead-plane )

Whilst most Obsidian is a natural stone, the beautiful Blue Obsidian is not, it is a man made glass.