You can't sharpen a stone point in the sense that you would sharpen steel to my knowledge. I'm sure you could use some kind of power tool to grind the edge pretty sharp but that's not knapping. To knap a sharp edge you want to use a nice sharp (pointy) flaker. Whether it be antler, bone, copper, steel, whatever and take tiny flakes off the edge back and forth. Kinda like what we call "stitching" the edge. That's about as sharp as your going to knap a rock..