Recently I've got the opportunity to challenge myself to sharp/carve, entirely by hand, Nixtamal* mill stones. - Essentials to make proper tortilla dough!
The stones are “tezontle negro“, a volcanic rock derive from magma. Currently the stones are brought from Mexico and become useless after a month of usage, new stones are required. The process of sharpening the stone will expand their life to about four times. I've also being in the search for this kind of stone in Japan to be able to produce the entire piece locally. 

I have heard that the "Picador de piedras de molino" ( mill stone hand-carver ) is a dying trade in México, it did not passed onto me by generations, but it will be great if I can contribute to its preservation and tribute.

* boiled corn and calcium hydroxide (slaked limestone) mix, ancient Aztec process called Nixtamalización.

- surrounding lava fields at Mont Asama.