The DMT 8 in Diamond Whetstone is a single-sided diamond bench stone in a hardwood box, in extra-coarse, coarse, fine or extra-fine.
The diamond surface is 8 in long and 2-5/8 in wide, room for knives, chisels and plane irons. It's interrupted by a pattern of recessed holes that collect the metal filings as you sharpen, so it doesn't clog. Sharpen dry or with water; no oil is needed.
It comes in a hardwood box with non-skid rubber feet. Use the extra-coarse to repair a damaged edge, the coarse to bring back a neglected one, the fine for a razor edge, and the extra-fine to polish and refine it. DMT says the extra-coarse is a common first step in flattening the backs of chisels and plane irons.
DMT, short for Diamond Machining Technology, was founded in 1976 by aerospace engineers. It started out making grinding wheels, and its first hand sharpener was made from the center punched out of a diamond grinding wheel. A buyer at L.L. Bean tested an early sample and worked with DMT's engineers to refine it. Acme United bought DMT in 2016, and DMT is based in Marlborough, Massachusetts.