Seastone

Kairoseki (海楼石), called Sea-Prism Stone in the English versions, and Cherry Stone in One Piece: Grand Battle, is a naturally occurring (though apparently rare) substance that can cancel out Devil Fruit powers. The Marines appear to have vast quantities of it, using it for jail cells, handcuffs, weapons and the bottoms of battleships. It first appeared in Chapter 100 and Episode 53.

How it Affects Devil Fruit Users
It was stated by Commodore Smoker that Seastone "emits the same energy as the sea", possibly in the form of special wavelengths of some sort. So far the material has only shown effect upon actual contact with someone who has eaten a Devil Fruit. Upon contact the material drains the victim's body of energy, and robs them of control of their Devil 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 Smoker can carry a Kairoseki-tipped jutte on his back, without it disabling him (presumably, thick clothing will prevent the Kairoseki's effect from harming the victim).

However, it has been shown that though a Devil Fruit user loses his powers when in contact with the Kairoseki, said powers can still be used on the Kairoseki, as shown seen when Mr. 3's candle wax key could open Kairoseki cuffs and cell doors, though this may be because the locks themselves aren't made of Kairoseki or because the wax has no Devil Fruit powers inside it as shown when he unlocked Ace's Kairoseki handcuffs. Another example is when Shiki levitated Battle Ships in Marineford, despite the hulls being lined with Kairoseki.

Other Effects
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 can't see the ship. Kairoseki is harder to break than diamond. This, along with the effect of Devil Fruits, is researched by Dr. Vegapunk, a Marine scientist.

According to Pagaya, an element contained in the Kairoseki called Pyrobloin form the special clouds (Sea Clouds or Island Clouds, depending on the density of the water) of the sky islands when it absorbs water in the sky.

Items
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 Fruit nullifier by the Marines as well as other groups like the Shichibukai and Cipher Pol.

Weapons

 * Nanashaku Jitte: Smoker's jutte, where the tip is made of Kairoseki, so Smoker can use it to nullify his opponent's Devil Fruit powers.


 * Wiper's Skates: Wiper's skate-type Waver has added Kairoseki, so he could use it to nullify Enel's Logia abilities.

Restraints

 * Kairoseki Net: Used in Loguetown, a net made of Kairoseki was thrown on Buggy and Alvida completely preventing them from using their powers. Alternatively, it was used to capture Monkey D. Luffy, Buggy, and Mr. 3 in Impel Down, but the Sphinx tore it apart, indicating that only parts of the net contain Kairoseki, while the rest is steel cable.


 * Kairoseki Cage: Seen in Alabasta, it was used by Crocodile to drain Luffy and Smoker's powers and make them unable to use their Devil Fruit abilities. According to Smoker, the Marines' central prison, Impel Down, also uses Kairoseki cages.


 * Kairoseki Cuffs: A special handcuff made to arrest those with Devil Fruit powers. The lock is made of Kairoseki so that when a Devil Fruit user is wearing them, they can't use their powers, but unlike other Kairoseki, it does not inhibit their mobility. During the Enies Lobby raid, Nico Robin wore them until Sogeking delivered the key to the bridge; despite being unable to use her powers to fight back, she was able to move, and could attempt to escape or otherwise resist being taken to the Gates of Justice. Sogeking tried to throw Kairoseki handcuffs at Jyabura and Kaku in order to prevent them from using their Devil Fruit powers and make them easier to defeat, but got distracted by Kaku's "square" giraffe face and accidentally threw them on Zoro's wrist, and got handcuffed to him while also ducking due to Kaku's Amanedachi. These were also hooked onto Ace's arms and legs in his prison cell in Impel Down to prevent him from using his Devil Fruit powers. Onigumo also clasped one onto Marco during the war in Marineford, to nullify his Devil Fruit powers.

Other Uses

 * Marine Ships: Dr. Vegapunk has discovered that 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.

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Kairōseki 海樓石