Shell and slate stones
Shell and slate stones manufactured in Japan are regarded by go players in Japan, China, and Korea as the finest stones available. Their weight, their color and grain, their pleasant feel, their responsiveness when placed on the board makes these stones the preferred choice for games played in international tournaments.
The black stones in these sets are made from slate nachiguro mined in Wakayama prefecture, Japan. The white stones are made from a clam called hamaguri. These clams used to be harvested in Hyuga, Miyazaki prefecture, on the southern Japanese island of Kyushu and processed there into stones. Because of the Japan Current the grain of the stones made from these clams is aesthetically more pleasing. However, because of overfishing, clam shells from Hyuga are scarce, so today almost all shell stones made and sold in Japan are from shells harvested in Lower California, Mexico.
Shell and slate stones are manufactured in thicknesses ranging from 7 to over 14mm. There is no standard thickness for stones; it is a matter of personal preference, but stones from about 8.8mm up to about 11mm thick have a nice feel to them. Aesthetically, if you are going to use the stones for a board with legs, the thicker the board, the thicker the stones should be.
Besides thickness, the white stones are classified into three grades: yuki, tsuki, and jitsuyo. Yuki stones have many grains which run straight across the top. Tsuki stones have fewer grains going across the top. Jitsuyo stones have either irregular or indistinct grain lines.