Bamboo charcoal (takesumi - 竹炭)

Bamboo charcoal is traditionally used in Japan for cooking and to dry tea. Its very high porosity is also used to purify water. A piece of bamboo charcoal left in the water of an electric kettle avoids any tarter to stick on the heating element. Japanese cook sometimes rice with a piece of bamboo charcoal to purify the water and add minerals. Rice cooked that way is meant to taste much better.

http://en.wikipedia.org/wiki/Bamboo_charcoal