Moon Pearl

A Link to the Past
In A Link to the Past, the Moon Pearl is a shining gem which allows Link to retain his normal shape while in the Dark World. Other humans entering the Dark World are transformed into shapes which reflect their "True Nature", or personality. This is shown by the pair of men on Death Mountain, as well as Link himself. The bully is transformed into a demon-like creature because of his devilish nature, and the other man turned into a ball because he could never make up his mind. Link's "true nature" is apparently a cute little pink bunny rabbit, probably due to his kindness and compassion. (This is commonly referred to as Bunny Link.)

Four Sword Adventures
In Four Swords Adventures, the Moon Pearl is a mysterious gem which is used to activate Moon Gates. In the game, they're said to guide its user into the Dark World, which is true since Moon gates are used to access the Dark World.

Unlike its predecessor in A Link to the Past, there appear to be more than one Moon Pearl, and while its function also seem to differs, they are both related to the Dark World. They are sometimes given to Link, who have the distinct abillity to sense Moon Gates, by civilians in hope of being saved. In dungeons they're often found in chests and are usually required to solve a certain puzzle.

When a Moon Pearl is brought nearby a unactivated Moon Gate, which is recognized by giving off a radiant glimmer, the pearl will glow as well. If brought too close, the Moon Pearl will burst, and a Moon Gate will appear. Because of this each Moon Pearl can only be used once and Link has to find a new Moon Pearl for each new Moon Gate.