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 12mm. However, most players agree that a thickness of about 9.2mm is ideal.
Yuki stones have clear grains which run straight across the top.
Tsuki stones have grain which curves across the top. Jitsuyo stones have either irregular or indistinct grain lines. Yuki grade shell stones with the grain highlighted. The photo on the extreme right shows the approximate color of the stones. In choosing a go set, the most important consideration is the balance in the quality of the board, stones, and bowls.