Spyderco manufactures small rectangular pocket stones for portable field use and honing small items.
Pocket Stones are available in fine grit (pictured), medium grit and doublestuff. The 303F Fine 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.
The Doublestuff is a 303F and 303M bonded together and packaged in a leather pouch offering a variety of sharpening options.