Atoma 400 Grit Diamond Plate, 210x75mm

These plates are famous for their very even diamond distribution, so abrasion is very powerful and consistent, and they are a little less prone to clogging because of the channels between the little piles of diamonds distributed on the surface. Great for knife and tool sharpening, but perhaps better known as being superb for flattening/lapping sharpening stones.

For lapping, also consider the much larger DMT Lapping stones which are specifically made for flattening stones: For coarse stones, for finer stones.