One Piece Devil Fruit

What is Devil's Fruit?
They are said to be the fruit of the Sea Devil and that the secret to their power is hidden in the Grand Line. Because encounters with them are rare (especially outside the Grand Line) a number of rumours about them have risen making it difficult to tell whether some things are fact or fiction. The average person knows nothing or very little about what they are.

The Devil Fruit, when consumed, gives the consumer a power or special trait. There are more than 100 types of Devil Fruit, [1] they come in different shapes, colors, and all Devil Fruits have swirl marks on them of some kind. Only one bite is needed for the power of a Devil Fruit to be yours, and after that, the Devil Fruit becomes a simple, useless, disgusting fruit. Although, it has been shown that you do not even need to bite or chew it. If you swallow it whole, as Buggy did, the effects are still the same. Modern science has also found a way for inanimate objects to gain the power of a Devil Fruit; this means Devil Fruit objects such as Lassoo the dog/gun and Funkfreed the elephant/sword can now exist.

While the storyline hasn't revealed it, there seems to be certain ways of telling what powers a Devil Fruit will grant you. Shanks and his crew already knew Luffy's Devil Fruit was the Gomu Gomu no Mi before he consumed it. Spandam mentions that the Devil Fruits he supplied to Kaku and Kalifa were not listed in any catalogs anywhere as to what powers they granted. He also mentions briefly that Devil Fruits have aura around them which can often help identify them and that the two Devil Fruits he had contained an unknown aura. Another example is Marshall D. Teach, telling Portgas D. Ace in chapter 440 that he had memorized all of the existing Devil Fruits shapes and "recognized" the Darkness one, thus knowing who he needed to kill to acquire the power he sought.

In the original two pilots of One Piece, Luffy's Devil Fruit power, the Gomu Gomu no Mi, was the power of the Gomu Gomu tree that grew once every 50 years and there were no signs there existed other powers. Many of the fruit names come from Japanese onomatopoeia and Japanese vocabulary.

Side Effects of Devil Fruits
With this new power comes a weakness, as the Devil Fruit user is rejected by the sea and becomes helpless while submerged in water (or "becomes a hammer," in Japanese parlance.) Oda stated in SBS that Devil Fruit users are susceptible to all types of water, not just seawater. However, if the user's body has been permanently altered by the fruit, then the user's ability can be manipulated by outside sources; i.e., when Luffy was knocked into the pool at Arlong Park, other people dove underwater and stretched his neck so his head would be above water. Being weak against the sea proves deadly to many Devil Fruit users who are pirates sailing the Grand Line.

One rumor, as recounted by Cipher Pol's Jyabura, is that Devil Fruits house actual devils that will fight when placed in proximity of one another. The resulting battle would destroy the users' bodies. Nevertheless, Blueno contradicted this claim, stating that Grand Line scientists have noted that the phenomenon of one's body destroying itself only occurs when one consumes two Devil Fruits.

Devil Fruit powers can be affected by Seastone or Blackbeard's nullifying powers.

The effects of Devil Fruit and seastone have been researched heavily by Doctor Vegapunk, a Navy Scientist. He also devised the method that lets an object (such as a gun or a sword) merge with a Devil Fruit.

Types of Devil Fruit
Paramecia - These fruits usually the affect the eater's body in some way.
Zoan - Allows the user to transform into a animal
Logia - With these fruits, the user may actually become a natural element.