To sharpen metal clipper blades, use a regular grindstone/whetstone from a hardware store. You want one with a grit rating of 3000 or 4000 for the initial grind, and then optionally an 8000 grit stone to finish. For ceramic blades, use a diamond sharpening stone. It’s important to check labels carefully. You can get ceramic sharpening stones that are designed for metal blades but not for ceramic blades. So buy a stone that is designed for your blades, not one made from the same material.