Yixing clay comes from Yixing town in Jiangsu Province, situated in the delta of the Yangtze River. It is a compound clay rich in minerals such as kaolin, mica, quartz and has high iron oxide content. Used almost exclusively for making teaware, it plays one of the central roles in Gong Fu tea culture. The umbrella name for all the Yixing clays is Zi Sha, which means "purple mud".
The unique mineral composition, the heat retaining properties, and air permeability that allows the clay to "breathe" enable this clay to interact with tea, transforming its taste beneficially (when used correctly).
Zi Ni is purple mud, one of the most famous and popular Zi Sha variety. This clay is the most porous one and has very good heat-retaining qualities. High porosity and mineral composition mutes tea's high/bright notes and makes the taste more rounded. Therefore, this clay usually pairs well with darker teas, such as Yancha and Pu-erh.
I purchased this tea pot in Beijing during a business trip in 1997.