Seastone is a strange and extremelt rare material that emits the energy of the sea. When Devil Fruit users come into contact with it, they experience the silmilar effects as when they fall into the sea.
Seastone is commonly applied in combat. For example, weapons could be tipped with, or entirly made out of seastone, to easier subdue Devil Fruit users during battle. It can also be used for handcuffs or chains to restrain Devil Fruit users.