The Crystal Idol is a term in Final Fantasy Crystal Chronicles: The Crystal Bearers. Crystal Idols are said to be royal treasures of the Lilty Kingdom, but, in reality, they are artifacts used to retrieve the crystal shards of the Yuke Crystal in order to "resurrect" it.
To activate a Crystal Idol, one must take it to the Subterranean Ruins's Crystal Chamber, near the Old Lett Aqueduct. The idol will then release all the crystals it has collected into the Yuke Crystal, fulfilling its duty, and changing into a powerless black crystal.
The three idols were sought by Amidatelion before and during the events of the game. She used the first one to steal the crystal of the Alexis. The second, (found by Belle) was used to steal the ones on the Aerial Prison. Princess Althea was shown to wear the third Idol at all times, until it was used to retrieve the remaining shards in Alfitaria, completing the Yuke Crystal's Resurrection and returning said tribe to the world of the living.