The Shapton Diamond Glass Lapping Plate (DGLP) is constructed from a substrate of glass coated with diamonds embedded in a nickel coating, so it will not rust like other lapping plates with steel substrates. It can be used to flatten synthetic and natural waterstones, ceramic stones, and oil stones that are 500 grit (30µm) or finer. The size range of the diamond abrasive used in this plate is 45µm to 54µm (270/325 mesh), so Shapton doesn't recommend using it to flatten stones with abrasive particles coarser than 30 µm because this will shorten the usable life considerably.
Although the DGLP is certainly not cheap, it is by far the best lapping plate that we know of. The lapping surface is incredibly flat at ±5 µm (0.005 mm or 0.0002") which is amazing for such a large surface, and several times better than other commercially available lapping plates.
The working surface of the DGLP has a special pattern of recesses designed to remove swarf and prevent stones from sticking/wringing (hydraulic/vacuum lock see https://www.youtube.com/watch?v=NGWHT7Q9WtA). The dimensions of the DGLP are 253 mm x 80 mm x 24 mm (9.96" x 3.15" x 0.95") with a working surface of 254 mm x 72 mm (10" x 2.83") and weighs about 750 g (1.65 lbs). It has a non-slip rubber base and it can be used in the field stone holder or on it's own.