All natural whetstones are different. Virtually every country in the world has its unique natural stone. Belgium has Belgian Blue Whetstones known as BBW. BBW has a high garnet content, and garnet crystals give it extraordinary properties. Usually sharpening grains must have sharp edges to be high-abrasive. Garnets have dodecahedron shape (ball with 20 vertices) which perfectly suits purpose of ultra-fine sharpening and polishing. Size of one garnet is 10-25 micron.
This is how BBW looks like in nature. You will notice a thin yellow layer inside the rock of brown BBW. It's Coticule, the rare and expensive whetstone, a satellite of BBW.
The surface of BBW has distinctive texture:
BBW is universal and can be used on stainless and high-carbon steels of any hardness and composition. BBW creates a very accurate cutting edge if pre-finish sharpening is made without flaws.
BBW is available on Gritomatic in Edge Pro format.
Courtesy of Mr. Wizard. Please read chart explanation in README.