Force Gems are the main power source of the Four Sword. They are collected in The Legend of Zelda: Four Swords Adventures much like Rupees in other games. Once enough Force Gems have been collected, the Four Sword will obtain the power to repel evil, turning it into a projectile sword and enabling it to destroy the end gate. However, if Link's Force Gems drop below the required amount, he will have to reacquire them before he can continue. If Link reaches the end gate without sufficient Force Gems, he will be teleported back to the start in order to acquire the Force Gems. He does not, however, have to go through everything again.
In The Legend of Zelda: Phantom Hourglass, Force Gems are much larger than in FSA, and are used in solving puzzles. In the Temple of the Ocean King, three must be arranged in certain areas to light up a Triforce symbol and open a door.
Much like rupees in other games, force gems are highly attractive to Tingle. If accessible, force gems must be collected before Tingle can grab them. If currently inaccessible, the player must quickly move to the next screen.