This crystal grindstone is made specifically for sanding urushi lacqureware, by well-known Japan grindstone manufacturer.  These are widely used by lacquerware artisans when traditional magnolia polishing charcoal is becoming difficult to obtain.   
 
Unlike regular whetstones and sandpaper, this special grindstone is easy on ceramics glazed surface while sanding down kokusou, sabi, and all the layers of urushi lacquer.  
 
My personal favorite would still be using charcoal to polish urushi.  But since charcoal is restricted from shipping, this crystal grindstone would be the best replacement.  These are resin-binded blocks that grind the surface evenly (unlike sandpaper), and during wetsanding it forms to the shape of your surface.  It also cuts relatively easy, similar to cutting charcoal. (see video) Just use a simple saw blade for stones.  
 
Drop the wet sandpaper in hobby kintsugi kit and use these grindstones instead.  You will not want to go back to sandpaper anymore. 
 
HOW TO USE?
It comes in a block and you can cut them to any size for your various grinding and polishing needs.   Mask the area with paper maskng tape.  Add water while sanding.  Keep the tiny ones even to the size of rice grain for very hard to reach places.  And the stone would wears out eventually.  
 
GRIT #
The grindstone comes in different grit size for different purposes.  
#400 for rougher sanding like kokusou
#800/1000 for sabi and naka  (middle coat) urushi
#2000 for shita, naka and ua-urushi polishing (base, middle and top lacquer coat polishing)
 
Basic Set of 3 includes 1 grindstone of each grit# (400, 800, 1000)
Just ADDED!  We now offer #2000 and #3000 grit, order as single item.  
 
PRODUCT OF JAPAN