Seastone

Seastone (海楼石 Kairōseki), 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.

First Appearance: Chapter 100 ;Episode 53

How it Affects Devil Fruit Users
It was stated by Captain 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 any 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 him of control of his 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 Captain Smoker can carry a seastone-tipped jutte on his back, without it disabling him. (Presumably, even thick clothing will prevent the Seastone's effect from harming the victim.). The sea stone is also said to be as hard as diamond.

Other Effects
The effect of seastones also affect creatures of the seas, such as Sea Kings. By layering the bottom of the ships with a layer of seastones, 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 can't see the ship.

It is recently discovered that Seastone is harder to break than 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. Thusly, it is deployed as a Devil Fruit restrainer by the Marines as well as other groups like the Shichibukai and Cipher Pol.

Kairoseki Cage
Kairoseki Cage: Seen in Alabasta, it was used by Sir 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 also uses Seastone cages.

Kairoseki Net
Kairoseki Net: Used in Loguetown, a net made of Seastone was thrown on Buggy and Alvida completely preventing them from using their powers.

Kairoseki Cuffs
Kairoseki 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 can't 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 them on 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 Jyabura and Kaku in order to prevent them from using their Devil Fruit powers and make them easier to defeat, but distracted by Kaku's "square" giraffe face, accidentally threw them on Zoro's wrist, and got handcuffed to him while ducking Kaku's Amanedachi.