Diamond plates for sharpening tools, putting a final edge on a knife or lapping a surface to ensure flatness. You get diamond pads with a plastic backing and a grid of holes or the diamond pads that are on a steel plate. I find that the best diamond pads are those on a steel plate.
For lathe tools I use a diamond plate to put the final edge on the tool. You can hold the angle accurately and very carefully get a high finish on the tool.
These steel backed diamond pads are also great if you want to sharpen a hard scribe that has been made from tungsten carbide.
I find the best place to purchase these diamond plates for sharpening is from Arceurotrade as they have them in 300, 600 and 1200 grit and a really useful size of 2 x 6 x 1/4 inch.