The world is full of companies manufacturing sharpening stones designed for outdoor use. Years ago Spyderco introduced an eight-inch stone in either fine or medium grit unequivocally designed for the field and range called the 701 Profile.
701MF stones are made of High Alumina Ceramic. A medium grit (brown) stone sharpens very dull edges. A fine grit (white) stone adds a finishing touch for a professional, finely-honed edge. Both stones are used dry without oil or water lubrication making them convenient and mess-free. Elliptical in shape, the Outdoor Profile stones have four distinct sharpening surfaces suitable for plain and serrated edges, shop tools and household items. They provide: A flat surface and large radius for PlainEdges, a small radius for SpyderEdges and a groove running the length of the stones for sharpening pointed objects like fishhooks, awls and darts. Each stone measures 3/4" x 8" x 1/4" (19mm x 203mm x 6.4mm).
Information from Spyderco