The ishi-usu is a Japanese stone hand mill made from two discs of granite with carved grinding surfaces. The upper stone rotates on a central pivot while material fed through a hole in the top is ground between the faces. For matcha production, the slow rotation (one turn per second) prevents heat buildup that would damage chlorophyll and amino acids. A quality ishi-usu produces matcha with a particle size of 5-20 micrometres.