the edge of a knapped head is a wavy line with some sections that are very sharp. However, because the edge is not straight and inevitably has some not-so-sharp sections, it doesn't feel as sharp as steel. This is especially true after it has been rattling around in your quiver. Also, the edge angle on stone heads is usually not as small an angle as on steel. So, obsidian can create the sharpest edge known, in practice it's not like a sharp steel edge. I do hunt with stone heads, and feel that they are "sharp enough".