It was stated by Vice AdmiralSmoker that Seastone "gives off a wavelength that is the same as the sea itself" and referred to it as the sea in solid form. It is also said to be as hard as diamond.
Because Seastone so strongly resembles the sea itself, Sea Kings cannot perceive it. By layering the bottom of ships with Seastone, creatures that are underneath the ship will be unaware of the vessels presence, and thus will not attack it. However, while the Seastone hides the movements of the vessel while it is traversing above the Sea Kings, that does not mean they cannot see the ship; if a Sea King comes to the waters surface, then it will be able to perceive the part of the ship that isn’t protected by Seastone.
According to Pagaya, Seastone contains a compound known as Pyrobloin (パイロブロイン,Pairoburoin?, Funimation: Pyrobroine), which is sometimes launched into the sky as "corneous particles" (角質の粒子,kakushitsu no ryūshi?) due to volcanic eruptions. In the sky, the Pyrobloin particles absorb moisture and form either Island Clouds or Sea Clouds, depending on the density of the water vapor.
Effects on Devil Fruit Users
Upon contact with a Devil Fruit user, Seastone will drain the Devil Fruit user of their stamina and nullify their powers. Coming into contact with Seastone has the same effect on a Devil Fruit user as if they fell into the sea, minus the risk of drowning. This weakening effect only works upon direct and constant physical contact with the Devil Fruit user in question, the effect stops immediately once the Devil Fruit user is no longer in contact with the substance.
Seastone can be modified to make a Devil Fruit user weak to only a certain extent; for example, Seastone handcuffs are generally modified to rob a prisoner of their powers, but still leave the prisoner strong enough to walk normally, and in some cases, perform hard labor. The degree of "weakness" induced by the Seastone depends on its density and composition, 100% pure Seastone having the greatest weakening effect on Devil Fruit users. Although, a Devil Fruit user can resist and even ignore the weakening effect if they have enough physical endurance.
It has been suggested that being weakened by Seastone affects a Devil Fruit users ability to use Haki, as Luffy was unable to use it with Seastone handcuffs on and became able to use it once again they were removed.
Although a Devil Fruit user loses their powers when in contact with Seastone, said powers can still be used on and affect Seastone, as long as the user themself is not making contact with the substance. An example is Mr. 3's Doru Doru no Mi allowing him to create candle wax keys that can unlock Seastone cuffs and cell doors. Another example is Shikilevitating battleships in Marineford despite the hulls being lined with Seastone, and Trafalgar Law was able to switch Seastone chains with regular ones using his swapping ability.
Because Seastone is as strong as diamond, and because it is such an effective countermeasure against Devil Fruit users, it is often used by various organizations to make a variety of weapons, such as blades or bullets, and restraints, such as shackles or cages.
Some of these items are adapted so that people with Devil Fruits can use them without actually making contact with the Seastone itself, a circumstance which would nullify their own powers in the process of trying to handle another Devil Fruit user.
Wano Country has many craftsmen who have expertise in working with the mineral and are the only ones skilled enough to forge it into delicate items, such as nails.
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.
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.
Prison Bullets (監獄弾,Kangoku Dan?): Prison Bullets are nets made of Seastone which were designed by the Marines to capture Devil Fruit users. They were first used in Loguetown by Marines to nullify Buggy and Alvida's powers after Smoker apprehended them. In the anime, these are shown being fired out of bazookas. A much larger net was used to capture Luffy, Buggy, and Mr. 3 in Impel Down, but a Sphinx captured with them tore it apart, indicating that only parts of the net contain Seastone, while the rest is steel cable. They were later seen being fired out of bazookas to capture escaping prisoners by Impel Down's Prison Bullet Bazooka Unit. They are called Jail Bullets in the Odex dub and Cage Shots by Funimation during the Loguetown Arc.
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 Summit War 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 was made out of Seastone.
Translation and Dub Issues
While Seastone is used almost universally among English-speaking fans, it technically abridges the original term Kairōseki (海楼石,Kairōseki?), by excluding the rō (楼,rō?) character between the "sea" (海,Kai?) and "stone" (石,Seki?) characters. This rō may be variously translated as "storey", "building", or "tower"; its particular significance to the material, if any, is yet unclear.
↑ 5.05.1One 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.