Crystal

From Zelda Wiki.org

The Crystals are magical prisons that hold the seven descendants of the Sages in A Link to the Past. The crystals are kept in the dungeons of the Dark World. Princess Zelda herself is held in the seventh crystal. After each of the dungeons, a crystal is received, which frees the maiden in it.

Crystals are also present in Ocarina of Time and Majora's Mask. After completing a dungeon, a warp appears that turns Link into a crystal and warps him out of the dungeon. Princess Zelda is held captive in a crystal in Ganon's Tower at the end of Ocarina of Time. Crystals also appear in Four Swords Adventures, where the maidens are once again held inside them.