Last, it should be noted that dual-sided Coticule/Belgian Blue whetstones are also sometimes available which have a layer of Belgian Blue stone on one side and a layer of Coticule on the other. Thus, these whetstones provide a softer, more coarse, grit on one side and a harder, finer, grit on the other. Plus, due to the inherent toughness of the Belgian Blue stone, these dual-sided stones do not require a substrate layer.
using an appropriate blade for the task – a thinner blade for more delicate work, and a thicker blade whenever a thinner blade is not required (e.g. a thinner blade might be used to cut fillets, butterfly steak or roast for stuffing, or perform Mukimono, while a thicker one might be used to slice or chop repeatedly, separate primal cuts of poultry or small game, or scrape and trim fat from meat or hide, as these actions would be more likely to cause unnecessary wear on a thinner blade.)
Vielsalm Coticule on the other hand generally occurs in much more narrow layers sandwiched between the slate layer and the Belgian Blue layer and thus, it is both less plentiful and more expensive than Belgian Blue stone. Also, Coticule is divided into different grades and sometimes displays blemishes on the surface due to its proximity to the slate layer. Furthermore, it is somewhat harder than Belgium Blue stone and, due to its brittleness, it is bonded to a substrate layer of hard slate prior to sale to prevent the stone from breaking during use.
Both Belgian Blue and Vielsalm Coticule are ancient stone layers found in the Belgian Ardenne Mountains with characteristics similar to both Novaculite and Siliciclastic sedimentary stone in that it is a metamorphic stone consisting of both gray and yellow volcanic ash mixed with tiny Spessartite Garnet crystals suspended in a clay matrix. However, due to its geology, both types of stone occur only in vertical seams sandwiched between two thick layers of bluish-purple slate and thus, they must be meticulously extracted mostly by hand. However, this type of extraction process is both very time-consuming and very labor-intensive and, quarrymen can only extract the stone for a few months each year due to inclement weather conditions. Consequently, both Belgium Blue and Coticule whetstones tend to be somewhat expensive.