The crystal necklace is an object owned and used by Sheeta in Laputa: Castle in the Sky. Sheeta's grandmother gave her the necklace and taught her the spells she needed to say in order to use its powers. The pendant of the necklace is a blue crystal made of the element volucite (aetherium in the Disney English version of the film), a rare element which glows when activated and which possesses certain magical powers. Sheeta's crystal is etched with the symbol of the royal family of Laputa.
Role in the Film
When Sheeta falls from the airship at the beginning of the film, her crystal activates and causes her to float down to the Earth without being harmed. She and Pazu later use the crystal to fall safely from the railway and float down into the mine below.
The crystal can also perform other magical tasks. When Sheeta activates it with a spell her grandmother taught her, the crystal awakens a Laputan robot and points the way to Laputa, which is hidden in the clouds above. When Muska tries to touch the crystal while it is active, it shocks him, and he realizes that only Sheeta can control it.
In the mine, Uncle Pom shows Sheeta and Pazu how the rocks hold traces of the volucite/aetherium element, and Sheeta discovers that her crystal is made of the same material. When Sheeta and Muska reach the core of Laputa, they discover a giant volucite/aetherium crystal there which keeps the island floating. Sheeta and Pazu speak a spell of destruction while holding her smaller crystal to prevent Laputa from falling into Muska's hands; both crystals work together to destroy most of the man-made structures on the island. Presumably, the crystal necklace is either destroyed or remains behind when Sheeta and Pazu leave Laputa at the end of the film.