Four Elements

The Elements are the crystalline forms of four energies the world of The Legend of Zelda series depends on. They were first shown in Four Swords, but their significance was not revealed until the later games Four Swords Adventures and The Minish Cap. They are based on the classical elements.

Four Swords
The Elements can be briefly seen in the introduction sequence of Four Swords, in front of the altar where Vaati is sealed. The upper parts of the Great Keys also are designed after them.

Four Swords Adventures
In Four Swords Adventures, the Four Sword Sanctuary lies in ruins and only a few remains lay on the Elements' pedestals. The Elements were introduced under the name "royal jewels". Link needed them in order to gain entrance to the Tower of Winds to save Zelda. They were in the possession of the spirits of four Knights of Hyrule, who were cursed to roam as Stalfos in the Dark World until Link freed them. By doing so, the Elements, which were also cursed and turned into gates to the Dark World, were cleansed.

The Minish Cap
In The Minish Cap, the importance of the elements was revealed. They are the source of power of the Four Sword, each element granting the wielder of the sword another "clone". In order to save Zelda and stop Vaati, Link had to find each element, scattered in four temples throughout Hyrule. Each time he found a new element, Link brought it to the Elemental Sanctuary to upgrade his sword.

Earth Element
The Earth Element is the first of the four Elements to be gathered in The Minish Cap. It is obtained in Deepwood Shrine in the Minish Woods, and is needed along with the Fire Element for Link to make the White Sword from Melari's Sword.

In Four Swords Adventures, it was called the "purple royal jewel" and was the fourth Element Link obtained.

Fire Element
The Fire Element is the second of the four Elements gathered in The Minish Cap, obtained in the Cave of Flames. This, along with the Earth Element, is needed for Link to create the White Sword from Melari's Sword.

In Four Swords Adventures, it was called the "red royal jewel" and was the third Element Link obtained.

Water Element
The Water Element is the third of the four Elements collected by Link in The Minish Cap. It is found in the Temple of Droplets. When its power was infused into the White Sword, it becomes the Empowered White Sword.

In Four Swords Adventures, it was called the "blue royal jewel" and was the first Element Link obtained.

Wind Element
The Wind Element is the fourth and final of the four Elements gathered in The Minish Cap. Once infused with the Empowered White Sword, it will become the legendary blade, the Four Sword. Although expected to be found in the Fortress of Winds after defeating Mazaal, it is not. The element is instead located in the Palace of Winds, among the Cloud Tops.

In Four Swords Adventures, it was called the "green royal jewel" and was the second Element Link obtained.

Trivia

 * The Fire Element bears some resemblance to the Goron's Ruby from Ocarina of Time.
 * The Wind Element bears some resemblance to the Kokiri Emerald from Ocarina of Time.
 * The Water Element bears some resemblance to the symbol on Farore's Pearl from The Wind Waker and the Tune of Currents from Oracle of Ages. Also it bears a resemblance to the three semicircle forms that make up the symbol on Nayru's Pearl and on the Tune of Ages from the same games respectively.
 * Originally, the Wind Element was purple and the Earth Element was green in Four Swords. But in Four Sword Adventures and The Minish Cap they seem to have swapped color.