Spyderco manufactures small rectangular pocket stones for portable field use and honing small items.
Pocket Stones are available in medium grit (pictured) and fine grit (see related item). The 303M Medium Grit Pocket Stone fits in the palm and is packaged in a leather carrying case. It is autoclave safe (ideal for dental tool sharpening) or can be cleaned with a plastic scouring pad and powdered abrasive cleanser.
