For most go enthusiasts, owning a nice set of thick snow grade (Yuki 雪印) Mexican (メキシコ産) clamshell go stones is their dream. After all, only 10% of all Mexican clamshell go stones ever produced qualify to be called snow grade, so it is a rare and enviable privilege to own such a set of Japanese manufactured clamshell go stones (日向特製蛤碁石).
But there is an even higher class of Mexican clamshell go stones, the gold grade (金印). The demand for excellence in this product is such that only one out of every 200 snow grade Mexican clamshell stones can qualify as a gold grade. This means that in a set of 180 white snow grade stones, there might not even be one stone which qualifies. It also means that such stones can never be mass produced. Instead, they are mainly selected and treasured by the go stone producers for their own collections and rarely available on the market. It is claimed that these stones are the closest thing in quality to similar sized go stones which command sky high prices — stones produced from old Japanese domestic clamshells (日向特産蛤碁石).
Of course, clams need many years to grow to an adequate size to provide a thick enough shell, and therefore stone thickness corresponds with rarity and value. The thickest and most expensive Mexican clamshell stone in production is size 50 and to see anything above size 37 is already quite rare.
The set shown in the picture above is a set of size 38 gold grade Mexican clamshell go stones. I have only seen such stones marketed on Chinese websites and have never seen a set in person. So I can’t really say how much better they are than snow grade stones of similar size. But certainly the prices quoted at these Chinese sites are phenomenal. The asking price for the set pictured above is a cool RMB138,000, equivalent to over USD20,000 at current exchange rates as of May 2010. At another Chinese site, there is a set of size 39 gold grade clam stones offered with an asking price of RMB300,000, equivalent to just under USD44,000 at current exchange rates as of May 2010. Certainly not equipment for those without deep deep pockets.
For more information about the grading of clamshell go stones, check out the English presentation from Kurokigoishi at http://www.kurokigoishi.co.jp/english/products/hamaguri.html