Horse pucky. All the major ones were carved with modern machinery in 19th Century Germany -- the research has been done to prove it.
The National Geographic says: "recent electron microscope analyses of skulls by the British Museum and the Smithsonian Institution revealed markings that could only have been made with modern carving implements. Both museums estimate that their skulls date to sometime in the mid- to late 1800s, a time when public interest in ancient cultures was high and museums were eager for pieces to display.
"Its examinations and the fact that no such skull has ever been uncovered at an official archaeological excavation led the British Museum to extrapolate that all of the famed crystal skulls are likely fakes."
Marks that could only be made by machine tools have also been discovered on many stones inside the pyramids.