Opinel are all about traditional methods - if something works why change it. So when it comes to sharpening their blades of course they recommend a natural stone from the quarries of Lombardy, Italy. They been using these stones since the 15th century.
The rock in Lombardy has the right balance of quartz and carbonates in the rock to provide excellent sharpening results. These stones are used with water and are perfect to regular maintenance sharpening. If you have a really blunt blade they you might need something courser to start the process with.
This smaller sized one is spot on for sharpening pen knives and if wish to carry it with you.
Opinel do offer a larger natural sharpening stone which you can find here.