The Crystal Skulls is a special feature contained on disc two of the two-disc DVD set of Indiana Jones and the Kingdom of the Crystal Skull.
Effects supervisor John Rosengrant from the Stan Winston Studio and his team first made computer models of many different variations of the skull and showed them to director Steven Spielberg who selected among them and gave some additional ideas like omitting the nose or changing the eyeholes. Together, they varied the design online until Spielberg was satisfied.
The "crystal" skulls were not made from crystal, but from a water-clear urethane in order to have less weight and to be more durable. They were cast hollow and received a special surface treatment to make them look more dense than they were. Different sizes of crystal skulls were produced for different scenes.
The thirteen crystal skeletons appearing at the end of the movie were nine feet tall and very slender. The bones were rather modeled after bird skeletons than after human bones to make the skeletons appear lightweight and fragile. The skeletons were cast from urethane, too. There were slight variations in the hue to give the individual skeletons slightly different personalities. The eyes of their heads could light up.
Rosengrant's team also made the thrones for the skeletons which had been designed by the art department of Guy Hendrix Dyas.