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Four different sets of go stones.
The pieces used to mark the moves on the go board are known as stones. They are made from quite a variety of materials. Traditionally they are black and white, but any two colours can be used as long as they are easily distinguishable.
There are two main shapes:
- Japanese and Korean stones are biconvex, that is lens shaped, with the same convex appearance on both sides.
- Chinese stones are flat on one side. Go/Weiqi stones from Yunnan, a province in the Southwest of China, are called Yunzi (云子).
- The most common material for moderately priced stones today is glass. Their workmanship varies considerably, with cheaper sets showing high variations in size.
- Traditional Japanese stones are made of slate (black) and shell (white) - see section below.
- Traditional Chinese stones are sintered (powdered and then melted together) jade (black) and sintered quartz (white).
- The cheapest stones are made from plastic. These are lighter, and to sensitive players don't "feel right in the hand". They are also prone to weld lines and other molding defects.
- Ing stones, common in Taiwan, are plastic with a metal center.
- Historically, in Asian countries, materials such as bone or wood have also been used.
- Gems, especially jade were used in old times, when go boards were decorated. Jade stones were commonly used in China's upper classes and are still used in some Chinese tournament games (e.g. Sonoda Yuichi versus Zhang Wendong, 1989).
- Go stones made from semiprecious stone are available from Algorithmic Artisan. See this review.
- Ceramic stones are available in China, especially near the "porcelain capital" Jingdezhen.
Should you want to buy shell and slate stones, be prepared to pay for them. The slate is (relatively) easily found, but the shell is (very) expensive. Shell stones are also judged to be 'better' (and more expensive) if there is a large number of visible lines on them, especially if the lines are straight.
Shell stones are often identified by "grade" (印). For stones made from shell harvested off the coast of Japan the grades are: Flower (Hana 花), Moon (Tsuki 月) and Snow (Yuki 雪), from lowest to highest. Snow grade are the whitest stones with the finest pattern (and thus the most expensive). Moon and flower or "standard" grade have broader lines and slight discoloration. For stones made from clams harvested off the coast of Baja California (Mexico) the grades are Standard (or utility) (Jitsuyo 実用), Moon (tsuki 月 ), and Snow (yuki 雪). Japanese clam stones are several times the price of stones made from Mexican clams.
The price of shell stones also varies considerably depending on thickness. Thickness is expressed in units of 号 ("gō"), a traditional Japanese measure of distance. 30号 is 8.0mm and 35号 is 9.8mm. It is quite difficult to find shells large enough to give stones 10 mm and above.
Traditionally, shell stones have been made from hamaguri ( Meretrix lusoria). Because these clams often don't reach the necessary thickness, go stones are sometimes made from the Giant Clam ( Tridacna gigas) instead. That species is classified as vulnerable, which has led to some concerns.
Players and collectors interested in semi-precious stones as material for their stones might also consider the interesting selection offered by a US specialist maker: http://www.algorithmicartisan.com/gostones/
 From top left to bottom right: Biconvex glass stones, Chinese style glass stones, small plastic stones, magnetic stones.
 For more information, see http://www.kurokigoishi.co.jp/online_shop/english/go/goishi.html. clam shell