What is "yunzi"

Yunzi ( Traditional: 雲子; Simplified: 云子; Pinyin: Yúnzǐ) refer to special weiqi (Go) pieces manufactured in the Chinese province of Yunnan. At various times in history they have also been termed yunbian (云扁) and yunyaozi (云窑子). Though technically Yunzi refers only to sintered stones made of "yunzi" material (the exact composition is a secret), the term can also sometimes connote stones which are single-convex of any material. Yunzi-style stones made of jade for instance, were often presented to the reigning emperor and his court in ancient China.