Sea Prism Stone, often called seastone, is one of the most mysterious and valuable materials in the world of One Piece. It is a rare mineral found in the depths of the ocean, said to share the same essence as the sea itself. This unique property gives it the ability to neutralize the powers of Devil Fruit users, weakening them as if they were submerged in water. When a Devil Fruit user comes into contact with seastone, their body loses its strength, their abilities are suppressed, and they are left vulnerable, though they can still move faintly if the amount of contact is minimal. Because of this, the Marines and other organizations frequently use seastone shackles, handcuffs, or weaponized tools to restrain even the most dangerous pirates.
Beyond its power against Devil Fruit users, Seastone also possesses another remarkable quality. When it is carefully crafted and polished, it can emit the same energy as the sea, allowing ships fitted with seastone on their hulls to sail across the Calm Belt without attracting the monstrous Sea Kings that lurk there. This ability makes it an invaluable resource for the World Government’s naval operations, as it provides safe passage where others would be doomed. However, seastone is extremely difficult to mine and forge, requiring specialized craftsmanship and knowledge, which is why only certain regions, such as Wano, are known to produce it at a significant scale.
The rarity and utility of seastone make it one of the most closely guarded resources in the world. It is both a weapon and a shield, capable of bringing balance to battles against otherwise unstoppable Devil Fruit powers and maintaining the fragile control of the seas.