Approximately 480 million years of interplay of pressure and temperature effects on sediments of volcanic ash and clays result in the transformation and formation of garnet crystals in slate layers in which the whetstone layers are found and mined. Ever since the times of Pliny the Elder (ca. 24 AD), the Belgian whetstones have been known in the pits at Vielsalm in the Ardennes in Belgium.

There are two types of Belgian whetstone, the yellow whetstone “Coticule” and the blue whetstone, called “De Blauwe Wetsteen”. These stones have a unique character, because, depending on the stone, they consist to a high percentage of garnets with a size of 5-20 microns (size of the stone particles).

The yellow stone contains about 35-40 % garnets and has a fine grit of 8000. The blue stone was not discovered as a useful stone until 1996/1997. Its blue colour is due to the iron oxide. It contains about 30 % garnets and has a fineness of 5,000.

Garnets follow the diamond, corundum and beryl in terms of hardness. They have the crystalline form of a rhombic dodecahedron, are thus shaped like a football and have corresponding faces and edges. This means that, due to their hardness they wear well, and on the other hand, they also polish finely due to their round particle shape. This is unique to the Belgian whetstone. It achieves excellent grinding as well as polishing results. By polishing, the sharpness of the knife increases many times, and it also stays sharper longer. 

The yellow stones usually only occur in very thin layers. Therefore, they are connected with a layer of slate to stabilise them - the grinding matrix. In general, the thicknesses of both types of stone are always different. This is due to the natural thickness of the veins.