Sumi

Solid charcoal pieces are traditionally used for grinding and polishing layers of lacquer. Different qualities and varieties of charcoal are made from different species of wood such as magnolia or camphor. Recently, the use of charcoal has been being replaced with synthetic whetstones and sandpapers which can be manufactured with better consistency at a lower cost. High-quality charcoal for lacquer work can be exceedingly expensive. Charcoal as a powder (sumiko) has other uses in lacquer work.

Literally: charcoal