The Belgian blue stone is a natural whetstone millions of years old, mined in the Ardennes Belgium, alongside the more well-known ‘coticule’ (there is only one company left in the world that produces these stones).
The Belgian blue (and coticule) have garnets naturally embedded in the stone, making them effective natural stones for sharpening knives, tools and razors (the coticule is however the preferred razor hone of the two). As you continue sharpening (and slowly begin to wear down the stone) you also expose new garnets in the stones surface, which continue the cutting action throughout the stones layers. As with all natural stones, different layers of any one stone may have different concentrations of the cutting ingredient across the surface (and each individual stone may have different cutting characteristics to each other).
When used with a slurry stone (smaller off cut Belgian stone) and water, these BBW whetstones will give a more course sharpening surface to work on, in turn speeding up the sharpening process, replacing the need for using multiple synthetic stones of varying grits (e.g.: thick slurry equals a more course stone – watery slurry is more fine – water on stone only (no slurry) is ultra-fine/polishing stone. For this reason water alone is best used with the BBW in the final stages (if using water alone – you can also carefully use varying degrees of pressure during the sharpening process to speed up the sharpening speed if you do not want to use slurry – not as effective).
Keep in mind, even with slurry the Belgian blue whetstone is still a high grit stone (and like all high grit stones) should be used on blades with a decent degree of sharpness already. If you allow your blades to become to blunt and try to use the Belgian blue (like any higher grit stones), it can be a long process.
Natural stones do not come with grit ratings, however the BBW is usually labelled as around 4000 grit. My experience is that they are much higher grit than this especially with water alone (which feels like marble) and I do on occasion use them after the coticule with water alone to finish (before stropping).
Overtime, as they begin to wear/dish (like every whetstone in existence) it will need lapping (flattening). Having a nice flat surface makes the sharpening process more effective.
As these are natural stones (like all natural stones), they can be slower to produce results than the synthetic man-made stones which are designed to exact specifications (equal grit across all layers and surface). However natural stones provide a sharpening action preferred by many, not to mention using a stone from the earth that has been used by civilizations for centuries has it own rewards.