This man made grinding stone or ink stone features a large well that separates into two circular wells, one larger than the other. You might choose to grind your sumi ink stick on one side and fill and block the other side. There is a small surface for grinding along the edge of the inkstone. This is a handy tool when sumi painting. It is made of stone.
Be sure to clean the stone completely with water right away once you've used it - once the sumi ink dries on the surface, it is extremely difficult to remove. The old dried ink will certainly interfere with making your next supply of sumi ink.