Crystal

Crystals are recurring objects in  series, first introduced in The Adventure of Link.

The Adventure of Link
In the backstory of The Adventure of Link, Impa gives six crystals to Link when the Triforce mark appears on his left hand. These crystals must be placed in the foreheads of statues found in the six palaces scattered throughout Hyrule and, once all crystals are in place, the barrier protecting the entrance to the Great Palace will disappear, allowing Link to retrieve the Triforce of Courage and awake the slumbering Princess Zelda.

A Link to the Past
In A Link to the Past, Crystals are magical prisons that hold prisoner the seven Maidens descendants of the Sages. The crystals are kept in the dungeons of the Dark World. Princess Zelda herself is held in the seventh crystal. After defeating each of the boss's dungeons, a crystal is received, which frees the maiden in it. All seven crystals are needed to get into Ganon's Tower.

Ocarina of Time and Majora's Mask
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 pink crystal in Ganon's Tower at the end of Ocarina of Time.

Oracle of Seasons
In Oracle of Seasons, Link finds Din imprisoned within a blue crystal before he faces Onox. During the battle, Onox uses the comatose Din to shield himself from Link's attacks. Link can repel the crystal with the Rod of Seasons.

Four Swords
Blue Crystals appear in Four Swords. Link must pick them up from their pedestal and carry them over to another pedestal. Upon doing so, certain devices will activate, such as a door opening to allow access to the next area. If Link drops a crystal and it shatters, he must start over again.

Four Swords Adventures
Crystals also appear in Four Swords Adventures, and the maidens are once again held inside them. When the crystal explodes, the free maiden will take the form of a fairy.

The Crystal Trap
In The Crystal Trap, Ganon traps both the Triforce of Wisdom and the Triforce of Courage inside a crystal by using a spell written in a Magic Scroll. However, since the Triforce of Courage rests within Link's heart, the young hero was encased as well, forcing Princess Zelda to go on a quest to find the three items necessary to free Link within just 24 hours before he is trapped in the crystal forever.

Eventually, Princess Zelda finds the Magic Honey, the Handprint, and the Magnifying Glass and uses them to shatter the Crystal, thus freeing Link and allowing them to search for the Spear, the sacred weapon that can defeat Ganon.