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 drain the energy of Devil Fruit users to the point where they can't consciously control their powers. Dr. Vegapunk has invented revolutionary ways to use this rare substance. The Marines appear to have vast quantities of it, using it for jail cells, handcuffs, weapons, and the bottoms of battleships.

Properties
It is stated by Pagaya that Seastone contains a compound known as Pyrobloin (パイロブロイン) which when separated from Seastone can help to create Island Clouds or Sea Clouds.

How it Affects Devil Fruit Users
It was stated by Vice Admiral Smoker that Seastone "gives off a wavelength that is the same as the sea itself" making him refer to it 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 Fruit. Upon contact the material drains the victim's body of energy, and stops them from controlling their Devil Fruit powers - having the same effect as throwing them into the sea.

The degree of "weakness" induced by the Seastone depends on its density and composition; for example, Seastone handcuffs rob the prisoner of their abilities, but still allow the prisoner to walk around normally. Seastone only works upon direct and constant contact, which explains how Smoker can carry a Seastone-tipped jitte on his back, without it disabling him (presumably, thick clothing will prevent the Seastone'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 Seastone, said powers can still be used on the Seastone, as shown seen when Mr. 3's candle wax key could open Seastone cuffs and cell doors, though this may be because the locks themselves are not made of Seastone or because the wax has no Devil Fruit powers inside it as shown when he unlocked Ace's Seastone handcuffs. Another example is when Shiki levitated battleships in Marineford, despite the hulls being lined with Seastone. Trafalgar Law was also able to switch Seastone chains with regular ones, using his abilities.

Other Effects
The effects of Seastone also affect creatures of the sea such as Sea Kings. By layering the bottom of the ships with Seastone, creatures that are underneath the ship will be unaware of the vessel, and thus will not attack it. However, while the Seastone hides the movements of the vessel traversing above the Sea Kings, that does not mean they still cannot see the ship. Seastone is as hard as diamond. This, along with the effect of Devil Fruits, is researched by Dr. Vegapunk, a Marine scientist.

Items
Typically, Seastone 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 counter by the Marines as well as other groups like the Shichibukai and Cipher Pol. Some devices are adapted so the Marines with Devil 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.

Weapons

 * Nanashaku Jitte: Smoker's jitte, where the tip is made of Seastone, so Smoker can use it to nullify his opponent's Devil Fruit powers.
 * Wiper's Skates: Wiper's skate-type Waver has added Seastone, so he could use it to nullify Enel's Logia abilities.
 * Battle Smasher: Zephyr's large mechanical arm is embedded with Seastone, allowing him to effectively nullify Devil Fruit powers.
 * Bullets: Zephyr uses them for long-ranged attacks against Devil Fruit users, such as Monkey D. Luffy. Those bullets not only wound Devil Fruit users, but also rob them of their strength and their ability to use their powers until they are extracted, making them very effective weapons.

Restraints

 * Seastone Net: Used in Loguetown, a net made of Seastone 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 Seastone, while the rest is steel cable.
 * Seastone 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. The Government's central prison, Impel Down, also uses Seastone cages.
 * Seastone Cuffs: A special handcuff made to arrest those with Devil Fruit powers. The lock is made of Seastone so that when a Devil Fruit user is wearing them, they cannot use their powers, but unlike other Seastone, 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 Seastone handcuffs at Jabra 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 Seastone 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: Nico Robin was weakened by a jewel (a Sea-Prism Stone necklace) in the filler Episode 142 when she tried to stop Ian from escaping the Rainbow Mist.
 * Fortification: Some people use it to fortify sensitive buildings and facilities, making them virtually indestructible. The SMILE Factory and the walls of the Corrida Colosseum in Dressrosa are made out of Seastone.

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