Zoro couldn't cut them and they are stated to be as hard as diamond. So how are Sea-Prism Stone items made?
In the Alabasta Arc, Smoker revealed that Seastone has "Sea Power" and therefore nullifies abilities just like salt water. In other words, vegapunk discovered its usage, this was revealed by Smoker and confirmed by Coby after Water-Seven Arc.
Vegapunk was able to coat ship bottoms with seastone, enabling them to pass through the calm belt without alerting the Sea monsters to their presence. He discovered a way which is yet unrevealed but confirmed by Coby back then to make tools, handcuffs, and weapons, out of seastone.