Yunzi
Yunzi (云子 Hanyu Pinyin: Yśnzi) are Go/Weiqi stones from Yunnan and shortly called Yunzi.
Traditional Yunzi Go stones are half-flat (single-convex or "one-side-convex"). Modern Yunzi are available both half-flat and in lens shape (bi-convex), similar to Japanese go stones. The bi-convex yunzi are described as 型新.
Chinese players like this for analyzing variations by turning the stones in question upside down. (e.g. cf. with the picture on Weiqi Tiandi, where 杨莲 Yang Lian seems to have turned around all stones)
See XuYing 徐莹 turning around the Yunzi too.
Yunzi have a thickness of 7 to 9 mm and a diameter of 21mm to 22mm. Black stones are slightly larger to make up for an optical illusion. Both are non-glossy with a matte finish.
When Yunzi are held against the light, they will appear dark green or soft white respectively, and show a beautiful translucency.
Modern Yunzi white stones are pure white, while traditional Yunzi white stones have a yellowish tint to them and are known as Old Yunzi.
Yunzi playing stones are the official standard of the Chinese Weiqi Academy. They are used in major national and international tournaments.
Manufacturer
The Yunnan Weiqi Factory
Video from the Yunnan Weiqi Factory (8:30 minutes in Chinese)
Some known sellers outside of China
The Go Gamestore (Canada, sells single-convex and bi-convex)
JW & S Hardy (Australia, sells biconvex and single-convex)
Yellow Mountain Imports (US, sells single-convex, bi-convex and Old Yunzi)
Go-Figure! (Europe, sells single-convex Yunzi)
weiqistore.com (Europe, sells single-convex and bi-convex Yunzi)
hebsacker-verlag.de (Europe, sells single-convex and bi-convex Yunzi)
jeudego.com (Europe French, sells single-convex and bi-convex Yunzi)
Oiling Yunzi stones
Yunzi stones can be oiled like slate stones. This process gives the stones a light sheen. Ready Set Go? offers a tutorial on this using machine oil. Other oils can be used
as demonstrated by ChiyoDad. To prevent the surface from getting sticky over time, the base oil component should be non-drying.
Now here comes the test: If holding against the light and you see voids, resp. gas bubbles in it, you may have glass stones. (That's what I was told later)
Actually yunzi stones are manufactured from other materials in a process that I can only guess is similar to glass making. The stones become glass-like but are denser and have "better" properties. Air bubbles should be considered normal in yunzi stones. See the discussion below for more details on yunzi.
Lead Contamination circa 2007
See ChiyoDad's Blog for information on the problem of lead contamination in some charges of Yunzi produced by
The Yunnan Weiqi Factory. If you bought contaminated stones from YMI, it seems you can
exchange them. For other vendors, YMMV.