Sometimes you have a tool or blade that needs sharpening, but the edge is tough enough that a typical whetstone or sharpening system won’t…er, cut it. Could be a fancy ceramic or stone blade, or maybe you have a knife made from an exotic metal like vanadium carbide steel. In these situations, you’ll find a diamond whetstone to be particularly useful.

Made by Japanese company Naniwa, these “Diamond Stones” look and feel much like regular whetstones, but can handle the hardest materials with both ease and speed, leaving behind a razor sharp edge while wearing only slightly in the process itself. Basically, if these stones can’t sharpen the blade in question, nothing can.

They’re made from a thick aluminum base with a 1mm-thick layer of super hard resin bonded on top, throughout which the diamond abrasive is suspended. Despite the thinness of that abrasive layer, the surface wears extremely slowly and is always revealing fresh diamond particles as you go, so the stone lasts quite a long time.

All the stone needs is a spritz of water before and during each sharpening, then left to air dry between uses. It does also come with a dressing stone in case the diamond surface needs to be cleaned of any deposits of waste metal particles from sharpening.

The Diamond Stones come in a variety of grit levels, with prices ranging from $141 to $181 on Amazon:

• 600 grit
• 800 grit
• 1,000 grit
• 3,000 grit
• 6,000 grit