This is the larger of the two sharpening stones that Opinel now offer for their knives.
Previously both sizes of Opinel sharpening stone came from a quarry in Lombardy, Italy. However, due to issues sourcing larger pieces of stone from that quarry this stone now comes from the Ariège region in the Pyrenees.
It's schistose sandstone, which has a uniform disruption of fine grit within it making it perfect for a sharpening stone.
This stone is 14cm long, 5cm wide and 1.4cm high. This makes it useable to sharpen most blades from folding knives up to large kitchen knives.
Opinel recommend using it with water but there's no reason why this couldn't be used as a oil stone either.
Prior to the development of ceramic wetstones where the grit levels can be specificed all knives would be sharpened on natural stones. Next time you are visiting a historic building have a look on the door steps and lintels of windows. You will often see "V's" worn into the stone where knives and swords were sharpened over the years. You can often see them on the external window lintels of churches, which often had high quality stone with a good level of grit for sharpening.
Opinel do offer a smaller sharpening stone which you can find here.