I have the stone already 2 years in my possession. Stone number 231 is a "Bergischerland Sandstein", 220 x 60 x 32.5mm, weighing 1132 grams. The Scratch resistance = 800 grams (Sclerometer) and its mass is 2.65 g/cm3. The stone is quite coarse and by using a diamond stone to flatten, it seems to get a finer structure but it increases the cutting speed. Diamond is so hard that the tops of the quartz crystals are flattened. IMHO, the actual grain is around 800 a 1000.