What is the most expensive clay?

Porcelain is generally the most expensive clay. There are potters in the far east that have their grandfathers put clay into the ground so it can develop mold and that will add to the plasticity and it will sit on the ground for 100 years or more. That could be considered the most expensive because of the rarity and the amazing workability but that isn’t a commercial clay. In terms of commercial, porcelain is generally the most expensive clay.