Logia

Logia is a type of Devil Fruit. It is commonly characterized by giving its user the ability to turn into a natural element.

Effect
With these fruits, the user may actually become a natural element, as well as generate and manipulate it. They are neither completely human nor element in their default state. Logia users can turn completely into their element if they want to. This is often used for crossing great distances fast, or avoiding blows should their weakness be discovered.

Strengths
Even in their default states, Logia users are near invincible (even to each other), taking no damage from physical attacks. For example, if their element is not solid, a blow can pass right through the user while he maintains the rest of his body, and can even hold onto objects at the same time. Even Aokiji's solid ice body will re-form instantly if broken. If the user trains their new powers, they can manipulate anything related to their element (such as Crocodile's sand power giving him the ability to absorb moisture, or Eneru's ability to use the heat from his lighting to reshape objects, via smelting). Logia users can tap unlimited amounts of their elements from their own bodies, and the "extracted" element can be manipulated however the user wants. Most Logia seem to have the ability to fly (with the probable exception of Aokiji and Blackbeard, as those aren't elements that float in nature) by turning their whole body into their element or just the lower half of the body (this is shown being done by Ace, Crocodile, Smoker, and more recently, Kizaru).

Logia is the most powerful of the three groups, as well as the rarest. With the exception of the three non-canon users (Honey Queen, General Gasparde and Simon), all Logia users seen so far are part of the most powerful organizations in the world, with Eneru being the only one that is not part of the Three Great Powers, though his abilities are easily comparable to them and his rank is similar as well as he was ruler of Skypiea.

Weaknesses
The main method of causing harm to a Logia user (aside from exploiting the effects of the sea and Seastone) are natural weaknesses which, when exploited, can ultimately lower or negate a Logia user's power — for instance, the electrically powered Eneru can be hurt by insulating substances like rubber which also makes the insulator immune to his attacks, and Crocodile, made of sand, can be struck if his body is dampened first. Also, Logia users tend not to have any defensive abilities apart from their ability to turn into an element and just let the attack pass through them, because of this, when this ability is overcome the Logia user usually has to dodge attacks.

Recently, Silvers Rayleigh has been shown to be able to kick aside the strike of a Logia fruit user, as well as injure the user with a sword. This is done through a power in One Piece known as Haki. One of the uses of this power involves the ability to sap a Devil Fruit power from a person. This is especially useful against Logia fruit users, since they will not be able to retain their normal elemental or non-elemental form. They become susceptible to being damaged by physical attacks, as well as being killed. A good example would be how Luffy, when under the effect of anothers Haki, will no longer have good resistance to physical harm. Luffy's body is made of rubber which normally renders him unable to be hurt by physical blows. If Haki is used against Luffy, then he will feel the pain of blunt attacks and take damage like a normal person would.

Other information
Logia is the most powerful and rarest of the three groups of Devil Fruit. Logia users cannot control their elements if it is in another state of matter, but they may use their surroundings to generate it. These Devil Fruits are also not limited to the classical elements: fire, water, earth, air, and aether, but includes physical states like smoke and sand. There are two types: tangible (something a person could hold like Sand or Ice) and intangible (something that could not be held like Smoke or Fire). Furthermore, their ability to move out of harm's way using their powers seems to serve as a defense-mechanism of sorts designed to save them from otherwise life-threatening attacks (i.e. getting cut in half), and if not trained properly (like Crocodile has), could be easily bypassed by non-lethal physical attacks. This was demonstrated by Luffy in Nanohana where he used a Gomu Gomu No Rocket to propel himself inside a restaurant, sending Smoker and Ace (both Logia-type users) flying.

Amongst these elements, there is a "unique" fruit: darkness, which has traits not found in other Logia fruit users.

List of Known Logia Fruit

 * Moku Moku no Mi (English Versions: Plume-Plume Fruit/Smoke-Smoke Fruit): Element of smoke.
 * Mera Mera no Mi (English Versions: Flame-Flame Fruit/Flare-Flare Fruit): Element of fire.
 * Suna Suna no Mi (English Versions: Sand-Sand Fruit): Element of sand.
 * Goro Goro no Mi (English Dub: Rumble-Rumble Fruit): Element of lightning.
 * Hie Hie no Mi: Element of ice.
 * Yami Yami no Mi: Element of darkness and gravity.
 * Pika Pika no Mi: Element of light.

Filler

 * Toro Toro no Mi: Element of some unknown liquid. (Movie 2)
 * Ame Ame no Mi: Element of semi-liquid and liquid candy syrup. (Movie 4)
 * Basa Basa no Mi: Element of paper. (Video Game)