These are usually the most expensive stones one can get, since each stone is handmade using mechanical grinding and polishing equipment. The black stones are made of slate, while the white stones are made from clamshells. Due to a single clam shell yielding only a limited number of stones (around three), white stones are expensive to produce. The clamshells used to make the white stones used to be farmed from Japan, but since the supply is dwindling, most clamshells are harvested from Baja California in Mexico. These stones have three grades, depending on the stripe or grain pattern of the white stones. All grades use the same uniform black slate stones. Shell stones may also rarely be made out of the giant clam (shako in Japanese) shell, as well as a clam shell that has a slight purple tint. Clamshell stones are cut as cylinders using a diamond coring drill, then professionally ground, sanded, shaped, and polished into the final product. Shell and slate stones are natural products that provide the ultimate in Go stone aesthetics, and even the lowest grade are far more expensive than the common plastic, glass, and ceramic stones. The price for a set of shell and slate stones ranges from US$200 to over $5,000 according to grade, thicker stones by large costing progressively more within each grade, especially within the Yuki grade.