Why can't Zoro cut Seastone?
Kairoseki is as hard as diamond, as stated in Chapter 400 p 11, so it is unlikely for Law to have cut it. However, it is a chain and nobody said that the chain consists just of Kairoseki. It is highly likely that at most every second chain link is effectively Kairoseki, since it is as hard as Diamond, you could barely made it into a chain without any other material and after all it is a rare material.