Royal Jewels

Jewels are quest items in The Legend of Zelda: Four Swords Adventures. in total there are four Jewels guarded by four Hylian Knights each Knight and jewel are differently coloured matching the four tunics of the four Links.

Some time before the events of the game the Knights became under control (presumably by Vaati or Ganon) and were transformed into Stalfos. Link defeated the four and used the power of the four jewels to open a way to the Palace of Winds.