Mikawa Nagura are small stones used in sharpening and honing. They are rubbed on a whetstoneMy Whetstones
Japanese Natural Awasedo finishing stones. Left to right: Ohira Asagi, Ozuka Mizu Tomae, Nakayama Mizu, Shobudani Mizu Asagi, Ozuku Asagi Synthetic Stones Maker Grit Type Notes DMT 120 μm/120 mesh Dia-Flat Lapping Plate Atoma 140 Diamond Plate Naniwa 220 Flattening Stone Shapton 320 "Blue-Black" Kuromaku Naniwa 400 Super Ceramic "Spash and Go" Atoma 600 Diamond Plate Naniwa 600 Nagura Dressing Stone Yoshihiro 1000 Professional Grade Toishi Shapton 2000 "Green" Ku...
, usually a Japanese natural finishing stone. They create a slurrySlurry
Slurry is the water on a [[whetstones|whetstone]] mixed with the particles rubbed free from the base stone or off of the nagura or [[Mikawa Nagura]] - a small stoned rubbed on the base stone to create slurry. You can also run a whetstone with a diamond plate to get slurry of just the base stone - the diamond doesn't come off so it doesn't particles of its own. The metal that comes off of the knife and turns the water dark as well as clogging up the surface of the whetstone is called "swa...
which has a different particle size and type than the base stone and are used in progression to make each nagura successively fine and sharper hones. Mikawa Nagura are typically use for straight razors, but Japanese swords are also honed using Mikawa Nagura.

They are sometimes called Asano Nagura for the Asano quality control stamp.

The progression that I use and seems to be common is Botan, Tenjyo, Mejiro, Koma and then finally the Tomo Nagura which is a nagura made from the same stone as the base or something similar. Many people skip a few of the nagura in the progression since they're strictly not all required, but it's a kind of ritual.