This crystal skull from the British Museum was once believed to have been ancient Aztec handiwork. Then, electron microscopes revealed that it's a fake from the mid- to late-1800s. In fact, according to National Geographic, the museum's "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 (around the world) are likely fakes." Of course, none of this should have any impact on whether you think the Indiana Jones movie sucked or not. From National Geographic:
Many believe (crystal) skulls were carved thousands or even tens of thousands of years ago by an ancient Mesoamerican civilization. Others think they may be relics from the legendary island of Atlantis or proof that extraterrestrials visited the Aztec sometime before the Spanish conquest…
Skulls were prominent in ancient Mesoamerican artwork, particularly among the Aztec, so the connection between these artifacts and these civilizations is apt.
"[I]t was a symbol of regeneration," says Michael Smith, a professor of anthropology at Arizona State University. "There were several Aztec gods that were represented by skulls, so they were probably invoking these gods. I don't think they were supposed to have specific powers or anything like that."