Gummi Blocks

Gummi Blocks are a material used primarily in the creation and modification of Gummi Ships. They are elastic and rubber-like in texture, and appear in many varieties. Each type has its own shape and properties and a wide variety of uses.

Kingdom Hearts Birth by Sleep
In Kingdom Hearts Birth by Sleep, the Gummi Blocks have a larger part in the game's plot. They are known as Star Fragments, and are how King Mickey travels the worlds. The Star Fragments also appear as star-shaped crystals, as opposed to the block-like, geometric shapes of typical Gummi Blocks. It is known, however, that King Mickey will not be the only character to come in contact with Star Fragments; both Ventus and Captain Hook have been shown to find one of King Mickey's Star Fragments.

Ansem Reports
According to the Ansem Reports, Xehanort caused Gummi Blocks to appear in Radiant Garden for the first time, and was also the first to study them. By this time, the Disney Castle world had already made use of this material, as King Mickey appeared in Radiant Garden in a Gummi Ship soon after. Unknown to Xehanort, by opening the door to the heart of Radiant Garden, he also opened the gateway allowing his world to interact with others.

A basic understanding of what Gummi blocks are can be inferred from the Ansem Reports. It appears that when the door leading to a world's heart is opened, the wall that protects that world collapses. Gummi blocks then appear in said world, as in the case of Ansem's study of the meteors, suggesting that the blocks are remnants of the protective wall of the world. The nature and origin of Gummi blocks is what allows them to travel to other worlds.

As mentioned in the Ansem Reports, Gummi blocks are "elastic" and "bond easily" when combined, as evident when creating Gummi Ships. The blocks themselves are very numerous and come in all shapes and sizes with properties attached to each. When added to Gummi ships, they provide advantages such as improved armor, increased acceleration and greater firepower. In both versions of Kingdom Hearts, specialized Gummi blocks can be used for other advantages, such as warp travel in Kingdom Hearts.

Obtaining Gummi Blocks
The primary way that Sora obtains Gummi Blocks is by destroying enemy Gummi Ships and obtaining blocks or blueprints from their remnants. Rarer blocks, however, can be found in chests throughout the worlds and dropped from certain Heartless. Later on in Kingdom Hearts, they can be purchased from Cid who, in a nod to his affinity with Airships, is an expert in Gummi engineering. Cid resumes his former work with Gummi Ships when Sora finds half of a Navi-G block in Deep Jungle.

Travel
Gummi Ships, space vessels made from Gummi Blocks, are the only known method of traveling between worlds, other than the use of a Corridor of Darkness, or Captain Hook's vessel.

Trivia

 * Although the Gummi ship became more of a mini-game in Kingdom Hearts II, the importance also increased. This involved the worlds new magical barriers (which in fact means, the goal of each world is to find an object that will open a new Gummi path) and even on board cutscenes with voice acting.
 * In Kingdom Hearts II, enemy ships are not made of Gummi blocks, but are in fact actual ships.