Crystal skulls are not new to the Indiana Jones Expanded Adventures. The so-called Mitchell-Hedges crystal skull is featured in the West End Games RPG supplement Indiana Jones and the Golden Vampires (1995). Matt McCoy featured a crystal skull quest that spanned four Indiana Jones novels starting with Indiana Jones and the Philosopher's Stone (1995). And finally, Tokyo Disneyland has an attraction entitled Indiana Jones Adventure: Temple of the Crystal Skull.

Indiana Jones and the Golden Vampires excerpt:
Lectures notes: Indy on the Crystal Skull
The "Crystal Skull" also know as the "Skull of Doom" as well as the prosaic "Mitchell-Hedges Skull" is a piece of solid quartz crystal, carefully ground and polished into the size and shape of a human skull. It was found in 1927 by Frederick Mitchell-Hedges, a British explorer and archaeologist, in the ruins of the Mayan city of Lubaantún in British Honduras. Although the actual discovery was said to have by Anna Mitchell-Hedges, his adopted daughter, Frederick is always credited with the discovery.

The Crystal Skull is alleged to have many supernatural powers. Mitchell-Hedges believed that the Crystal Skull was used by Mayan priests in their rituals, and that death was somehow always connected to the Skull. He asserted that a Mayan high priest could, by concentrating on the Skull, bring death to someone else. The Skull is also alleged to have certain divinatory powers; if someone of the appropriate psychic temperament gazed at the surface of the Skull, then visions of the past or future would appear. It is also rumoured to produce a number of inexplicable phenomena: those in its vicinity have allegedly seen it glow brightly, or even create a sharp singing noise. Some would like to believe that the Crystal Skull is Atlantean in origin, actually predating the Maya. Remember, however, that archaeology is fact, not speculation!

Archaeologists have yet to fully agree upon what the Crystal Skull's real origins are. The skull is a common motif is Mesoamerican art, and a number of "crystal skulls" have been found, and their origins authenticated. This Crystal Skull, however, is rare in the perfection of its design; most other artifacts of a similar nature are often little more than stylized skulls, while this one is a perfect replica of a human skull, complete with hinged movable lower jaw.

Mitchell-Hedges has kept the Crystal Skull among his belongings. Although he has not released it to a museum - where I personally believe it belongs - he has been willing to let authorities examine it whenever possible, particularly if he thinks that such actions will help validate his claim. Why the Mexican government has not attempted to claim it as a national treasure is beyond me.

Indiana Jones and the Philosopher's Stone excerpts:
It was the size and shape of a genuine skull, rendered chillingly lifelike by some Iong-dead artist's attention to detail, including the zygomatic arches of the upper cheeks and an articulated lower jaw with a perfect set of teeth. And, Indy noted from the brow ridges, the skull was feminine.

"Ah, behold the unnamed goddess of death - ancient beyond imagination. Such extraordinary craftsmanship. Note the anatomical detail, Dr. Jones. She is quite unlike anything the Maya ever produced. They had no sense of the mimetic." He gazed wistfully into the depths of the skull. "No, this is the work of a previous and unknown civilization, one with the skills far superior to the Maya - and daresay, to our own

"What is the curse of the Crystal Skull?"
"Don't you know, boss? You will kill what you love."

Crystal Skulls - Fact and Fiction thread