Granite Stone Grinder
A stone mill consists of two granite parts. There is a slight opening in the middle where the two parts meet. This is called "Fukumi." The tea is funneled in from the top and fills in this small space, then is pushed out gradually. Because of this, the granite parts never touch each other during the grinding process.
There are grooves carved into varying grid lines patterns inside the mill which is designed to push the tea outward from within as the mill starts grinding. And by the time leaves finally reach the outer seam, they are broken down and ground into ultra fine powder. An axis made of oak wood is placed in the center to hold the mill together. This is also chiseled and shaped by the artisans to fit perfectly centered into each mill since no hole is ever drilled the same way when the granite arrives. Oak is ideal for the core for its strength and durability and has very little inherent smell.
Please watch the short animation that explains how Tencha is ground into Matcha.