Why can't Zoro cut Seastone? In chapter 402, page 8, Zoro and Usopp got cuffed together with a Seastone cuff, but Zoro didn't cut it, and instead he suggested to cut one of their hands. 

At first I thought that maybe it's impossible to cut Seastone, but then I read chapter 677 page 9, where Law can easily cut another person's Seastone cuff (he is the only one that get caught by normal cuff). So why didn't Zoro cut his, was it because he not strong enough?

Zoro



Law
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.