Seastone (海楼石,Kairōseki?, translated as "Seastone"; literally meaning "Sea Watchtower Stone"), 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.
It was stated by Vice AdmiralSmoker 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 move their bodies 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 although a Devil Fruit user loses his/her powers when in contact with the Seastone, said powers can still be used on and affect the Seastone. An example is 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 Shikilevitated 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.
According to Caribou, Seastone restraints can be modified to make a Devil Fruit user weak to a certain extent. Luffy has indicated that being weakened by Seastone affects his Haki, as he was unable to use it with Seastone handcuffs on and became able to once he had the cuffs removed.
The effects of Seastone also affect creatures of the sea such as Sea Kings. By layering the bottom of 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 cannot see the ship. This, along with the effect of Devil Fruits, is researched by Dr. Vegapunk, a Marine scientist.
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.
Wano Country has many craftsmen who have expertise in working with the mineral and are the only ones capable of using it to make delicate items, including nails.
Battle Smasher: Z's large mechanical arm is embedded with Seastone, allowing him to effectively nullify Devil Fruit powers.
Bullets: Z 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.Capone Bege threatened to shoot down Caesar Clown with such bullets if he did not show himself, only to reveal he did not have such bullets.
Spear: Bege has a spear made of Seastone which he had one of his subordinates use in order to restrain Caesar Clown.
Nails: A member of the Beasts Pirates who serves as a subordinate to Basil Hawkins has access to nails made of Seastone that he fires off from a gun. It was used on Trafalgar Law to take away his strength and Devil Fruit ability, having essentially the same effect of Seastone bullets.
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 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 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. Boa Hancock was required to wear one while visiting Ace at Impel Down. Onigumo also clasped one onto Marco during the Battle of Marineford, to nullify his Devil Fruit powers.Usopp fired a pair of Seastone shackles to coil around Caesar Clown's neck, disabling his Gasu Gasu no Mi ability.
Seastone Chains: Caesar had several Seastone chains on Punk Hazard, however, Law replaced some of them with regular ones. The chains were used to bind Law, Luffy, Smoker, and the rest of their party in a cage to be used to show Shinokuni power. The Prisoner Mine in Udon has access to 100% pure Seastone chains. They were used by Queen's subordinates to restrain Big Mom after she fell asleep.
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 Seastone necklace) in a filler 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 in Dressrosa is made out of Seastone.
↑ 11.011.1One Piece Manga — Vol. 93Chapter 935,
A Udon prison guard mentions how their prisoners with Devil Fruit powers are restrained with just enough seastone to allow them to perform manual labor.
↑ 12.012.1One Piece Manga and Anime — Vol. 53Chapter 516 and Episode 410,
the Kuja pirates spot a dead Sea King and Momonga notes though they can move above the Sea Kings nest unnoticed, it does not hide them from the creatures' sight.