For years the mysterious crystal was thought to be "Aztec Mexico" and dated tentatively "fifteenth century."

The most suspicious thing is that more crystal skulls have not been found. One is in a private California collection, and one is on public display in the Musee de l'Homme in Paris.

Some claimed the skulls represent the powerful god Mictlantecutli ("Lord of the Underworld"). The rock-crystal skulls of Paris and London are almost perfect copies of real human skulls.

George Frederick Kunz the minerologist who examined the skull in the 1880's writes in his book, Gems and Precious Stones of North America: "It was brought from Mexico by a Spanish officer sometime befor the French occupation of Mexico, and was sold to an English collector, at whose death it passed into the hands of E. Boban, of Paris, and then became the property of Mr. Sisson." when the Mexican art market was hardly profitable enough to warrant carving fakes from materials as expensive as rock crystal. (Sisson sold the skull to the British Museum in 1898 for less than two hundred pounds.) If a sculptor wanted to produce a fake, it would be more reasonable to copy a genuine pre-Columbian piece than to invent a "new realism" of his own.

Kunz never doubted the work was Aztec, but he wondered where such a large block of crystal could have been found. He speculated that it may have come from Calaveras County, California, "where masses of rock crystal are found containing vermicular prochlorite inclusions identical with those observed in the large skull."