Kairoseki (海楼石 Kairōseki?, translated as "Seastone"; literally meaning "Sea Watchtower Stone"), also being called Sea-Prism Stone, is a naturally occurring (though apparently rare) substance that can cancel out Devil's Fruit powers and weaken Devil Fruit Users. There has been revolutionary ways invented on the use of this rare substance. The Marines appear to have vast quantities of it, using it for jail cells, handcuffs, weapons, and the bottoms of battleships.
How it Affects Devil Fruit Users
It has been said that Seastone "gives off a wavelength that is the same as the sea itself" making it being refered to as a solid form of the sea. So far the material has only shown effect upon actual contact with someone who has eaten a Devil's Fruit. Upon contact the material drains the victim's body of energy, and negates their Devils Fruit powers - having the same effect as throwing them into the sea.
The degree of "weakness" induced by the Kairoseki depends on its density and composition; for example, Kairoseki handcuffs rob the prisoner of their abilities, but still allow the prisoner to walk around normally. Kairoseki only works upon direct and constant contact, which explains how Devil Fruit Users can carry around Kairoseki weapons that do not actually come into contact with them.
However, it has been said that though a Devil Fruit user loses his powers when in contact with the Kairoseki, said powers can still be used on the Kairoseki.
The effects of Kairoseki also affect creatures of the sea such as Sea Kings. By layering the bottom of the ships with Kairoseki, creatures that are underneath the ship will be unaware of the vessel, and thus will not attack it. However, while the Kairoseki hides the movements of the vessel traversing above the Sea Kings, that does not mean they still cannot see the ship. Kairoseki is harder to break than diamond. This, along with the effect of Devil Fruits, is currently unknown who actually researched upon these things.
Typically, Kairoseki is primarily found in the form of restraining devices, such as shackles or cages, but they have other uses. Thus, it is deployed as a Devil's Fruit counter by the Marines as well as other groups like the Shichibukai and Cipher Pol. Some devices are adapted so the Marines with Devil's Fruits can use these weapons, without actually making contact with the device themselves, an event which would nullify their own powers in the process of trying to handle another with a Devil Fruit ability.
- Marine Ships: Kairoseki can mask the presence of a ship from Sea Kings and other sea-dwellers, and therefore had the bottom of Marine ships covered in a layer of it, which is especially useful when traveling across the Calm Belt. However, the Sea Kings can still see the ship, so it is not an absolute protection.
- Jewelry: Kairoseki is used within rare and precious jewelry all throughout the world of Hedgehog Piece.