The Mayan Indians, 2,500 years ago, were said to have used obsidian blades, which are said to be five hundred times sharper than steel. But, why is obsidian so sharp? Its smooth edge is visible under a high magnification microscope, while steel blades have a saw-like edge. There are two possible explanations. The first one is that obsidian is much harder than steel, and the other is that the stone can fracture down to a single atom.
The second theory explains that diamond is the sharpest blade in the world. However, this is not true. This is because diamond crystals are so crystalline that they tend to cleave in an octahedral pattern. That makes it difficult to create a razor blade with a razor-like edge. This is where the most common steel and carbon steel comes into play.
The third theory is that obsidian is the sharpest blade in the world. These knives are so thin that they are only three nanometres wide at the edge. While diamond is the most hard substance on Earth, diamond crystals tend to cleave in an octahedral pattern, making it impossible to make a sharp knife cross-section. The sharpest blade in the universe would be made from carbon steel.