Seastone

Seastone (海楼石 Kairōseki), called Sea-Prism Stone in the English anime, and Cherry Stone in One Piece Grand Battle, is a naturally occurring (though apparently rare) substance that can cancel out Devil Fruit powers.

How it Effects Devil Fruits
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 robs the prisoner of their abilities, but still allows the prisoner to walk around. 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.)

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's underneath the boat will be unaware of the ship, and will not attack it. It is recently discovered that Seastone is harder to break than diamond. This, along with the effect of the Devil fruit, is researched by Dr. Begabank, a Navy 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 Net
Kairoseki Net:Seen used in Logue Town, a net made of Seastone was thrown on Buggy and Alvida completely cancelling out their powers.

Kairoseki Cuffs
Kairoseki Cuffs: A special handcuff made to arrest those with Devil Fruit powers. The lock is made of seastones so when a devil fruit users are wearing them, they can't use their powers, it slowly drains the person's strenght but unlike other seastones, the person won't be deadweight until the cuffs are off. During the Enies Lobby Raid, Nico Robin wore them until she was free of it on the bridge.