I made this tool out of a one-tooth section of a carbide circular saw blade:
formatting link
made it years ago to dig out ceramic tile grout. I worked OK for that, but tonight I found that it's real calling is as a scriber. The tip is not only very hard and very sharp, leaving a distinct and fine line, but from the side you can see that it can get very tight to the straight edge:
formatting link
Yeah, I know: you can buy carbide scriber's. But what's the fun in that?Bob