The Crystal Skulls

No crystal skull in a museum comes from a documented excavation, Jane Walsh wrote in 2008. That year a British Museum and Smithsonian study found the London skull cut with rotary wheels, the Smithsonian's with carborundum, a 20th-century abrasive.

Jane MacLaren Walsh, 'Legend of the Crystal Skulls', Archaeology 61:3 (May-June 2008); Margaret Sax, Jane M. Walsh, Ian C. Freestone, Andrew H. Rankin and Nigel D. Meeks, 'The origins of two purportedly pre-Columbian Mexican crystal skulls', Journal of Archaeological Science 35 (2008), 2751-2760, as reported by Paul Rincon, BBC News, 22 May 2008

Quartz carvings of human skulls, sold or claimed as ancient Aztec or Maya artefacts.

Sources4 sources

Scholarship

Jane MacLaren Walsh, Legend of the Crystal Skulls, Archaeology 61:3, 2008No museum crystal skull has a documented excavation; the London and Paris skulls tie to Mexico only through the dealer Eugène Boban.
not a single crystal skull in a museum collection comes from a documented excavation
Paul Rincon, Crystal skulls 'are modern fakes', BBC News, 2008, 22 May 2008Reports the joint British Museum and Smithsonian study in the Journal of Archaeological Science: rotary-wheel tooling on the London skull, carborundum on the Smithsonian skull.
The suggestion is that it was made in the 1950s or later
Margaret Sax, Jane M. Walsh, Ian C. Freestone, Andrew H. Rankin and Nigel D. Meeks, The origins of two purportedly pre-Columbian Mexican crystal skulls, Journal of Archaeological Science 35:10, 2008, pp. 2751-2760The 2008 joint British Museum-Smithsonian study itself (bibliographic details confirmed via Crossref; findings read in the BBC report, not in the paper).

The tradition itself

Chris Morton and Ceri Louise Thomas, The Mystery of the Crystal Skulls: Unlocking the Secrets of the Past, Present and Future (publisher's description), 1997The thirteen-skull prophecy as the book presents it: reunited at a time of great crisis to reveal what humanity needs to survive.
all thirteen crystal skulls would be rediscovered and brought together