Atoma 400 grit diamond plate with metal backing. With stone holder.
This stone was manufactured as a medium-coarse sharpening stone, but we much prefer it as a flattening stone. It especially is great for flattening finer finishing stones, especially natural finishing stones. This is our go-to for flattening natural finish stones here in our shop, as we are using a number of different natural stones weekly. Natural stones will generally not need as much flattening done as a synthetic stone.
Natural stones should still be smoothed over with a natural nagura after using this flattener. The deep grooves made by the diamond plate in the flattener will reduce the efficiency of the natural grit stones if they are not smoothed away. However, the slurry made from flattening and using nagura can be kept atop the stone to speed the polishing process.