The mysterious skulls in Indiana Jones and the Kingdom of the Crystal Skull are made from quartz crystal.
The film’s plot revolves around crystal skulls associated with an ancient civilization and a hidden city in the Amazon. Indiana Jones and his allies investigate the skull’s origins while Soviet agents pursue it for their own purposes.
The story uses the popular crystal-skull legend as a fictional adventure premise. In real archaeology, supposed crystal skull artifacts have been heavily disputed, and several famous examples have been identified as modern creations or objects made with modern tools.
Quartz crystal is therefore the film’s stated material, not a claim that the movie’s supernatural history is accepted archaeology. The distinction between cinematic mythology and archaeological evidence is important.