Crystal

Crystals are used to keep the Seven Maidens imprisoned in each of the dungeons of the Dark World in A Link to the Past. After beating a temple, Link would receive a crystal and free a maiden. After collecting all crystals, the barrier in front of Ganon's Tower is removed.

Crystals also appeared in Ocarina of Time and Majora's Mask, where they are used to warp Link out of a dungeon after completion. At the end of OOT, Princess Zelda is held hostage in a crystal. Crystals also appear in Four Swords Adventures, where they are keeping the maidens sealed once again, and are being plagued with Vaati's energy, requiring the player to collect enough Force Gems to destroy the magic.