Call of the Crystal is the eerie theme for the crystal skull of Akator from Indiana Jones and the Kingdom of the Crystal Skull. It was written by John Williams, and appears as the second track on the soundtrack album, in suite form.
Due to the similar nature and subject of the fourth Indiana Jones film, Williams chose to write the theme as somewhat of an homage to the creepy, otherwordly scores of science fiction movies of the 1950s. The leitmotif utilizes a theremin, an early electronic music instrument which produces a strange and mysterious sound. The instrument was popular in Hollywood science fiction movies such as Bernard Herrmann's 1951 score to The Day the Earth Stood Still.
The theme also makes use of brass instruments, to give it some power.
The tone of the skull's theme is a combination of deranged anguish and frightful mystery. An ascending, spellbinding three-note progression is repeated for the main part of the piece, followed up by an upper range six-section. Played together, they seem to oppose one another, and their incongruous connection gives a mysterious if frightening feel to the music.
The latter portion is mainly of a rising, falling, and rising pattern, with a rather sad, desperate mood, effectively giving the impression of an undeniable call. It sounds as if it is crying out to be heard; this calling aspect is an important characteristic of the crystal skull in the movie, as its alluring, hypnotic powers call those who seek it to its rightful place. This appeal builds to two crescendos in the piece.
Toward the beginning, and later at the end of the piece, another, more subtle motif is introduced, a section with an almost circulating sound. In the latter half of the movie, this motif is revealed as symbollic of the interdimensional beings to which the skull belongs. It appears for the final time toward the end of the album track The Departure, which corresponds to the movie as the flying saucer lifts from the ground with grand, ethereal musical accompaniment.
As the theme to the crystal skull, the piece can be compared to the theme for the Ark of the Covenant, which was originated in the score to Raiders of the Lost Ark, also by John Williams. As the Call of the Crystal features an ascending three-note motif, the Ark Theme highlights a three-note descending motif.
The first time the crystal skull theme is heard in the film is when Indiana Jones explains the origins of it and the legend of Akator to Mutt at Arnie's Diner. It is heard for the last time as the saucer leaves the earth, in an excited manner. The theme does not appear in the ending credtis, as the Ark Theme or the Grail Theme in Indiana Jones and the Last Crusade did.