Coarse Atoma 140 diamond plate.
This stone was manufactured as a coarse sharpening stone, but we much prefer it as a flattening stone. It especially is great for flattening hard ceramic stones. This is our go-to for flattening here in our shop, as we are using a number of different stones all day. Fine stones (3000 grit plus) should be smoothed over with a nagura after using this stone. The deep grooves made by the coarse diamond plate in the flattener will reduce the efficiency of the finer grit stones if they are not smoothed away.