It is a fumigated silver leaf with sulfur that discolor into BLACK by the sulfurization reaction. Silver is discolored into red, blue, and black by smoking in sulfur. The technique of smoking the silver leaf by sulfur is originally developed in Japan. It is a very special skill that can be done by limited craftsmen. Once discoloring into black, the leaf itself becomes very fragile and way too soft. That is why, it has not been used commonly. However, it was discovered to be adequate as practical use in 2016 and now is used in the many different products.