Several crystal skulls have been found in Mexico, Central and South America, around the ruins of Mayan or Aztec civilizations. They are supposed to have magical or healing powers. Their age is hard to discover owing to the impossibility of carbon dating crystal; one website that I visited claimed their age was between 5000 to 36000 years old.
The workmanship is extremely puzzling. Many are highly intricate, and analysis by lapidaries indicate that since they were carved against the grain it is a marvel that they did not shatter when created, or in the time since. Some of the more interesting skulls found are listed here:
Mitchell-Hedges skull: The most famous of the crystal skulls, this one was so finely carved that some have hypothesised that it was cut with diamond tools and smoothed with sand and water.
British Crystal Skull, aka Aztec Skull: Bought by mercenaries in Mexico in the 1890s, possibly along with the Paris Crystal Skull, it now resides in the Museum of Man in London. It is formed of cloudier crystal than the Mitchell-Hedges skull, and is not so finely carved. Also, the jawbone is attached, rather than individually carved. Some museum workers and visitors have claimed to see this skull move inside its glass case.
Paris Crystal Skull: The similarity to the British Crystal Skull leads some to believe that one is a copy of the other. It resides in the Trocadero Museum in Paris. There is a hole in the top of the skull, purported to have held a cross.
Mayan Crystal Skull and the Amethyst Skull: Discoverd in Guatemala and Mexico respectively in the early 1900s and brought to the US by a Mayan priest. Very similar to each other, the only notable difference is that the Amethyst skull is carved of purple quartz while the Mayan is clear. Both skulls were examined at Hewlett-Packard and found to be cut against the axis of the crystal.
Smithsonian Crystal Skull: A 40-pound hollow rock quartz skull owned by the Smithsonian Institution, but not on display, due to lack of any knowledge of its age or origin.
Texas Crystal Skull or 'Max': A clear, single-piece skull supposedly from Guatemala, it came into the possession of Norbu Chen, a Tibetan spiritualist. It is now owned by JoAnn Parks of Houston, Texas.
ET Skull: A skull with a pointed cranium and an overbite, it is owned by Joke Van Dietan (author of Messengers of Ancient Wisdom), who believes it to have healing powers.
Rose Quartz Crystal Skull: The second most detailed skull after the Mitchell-Hedges skull, it also boasts a detachable jawbone. Found near the border of Honduras and Guatemala.
Sha-Na-Ra and The Rainbow Skull: Found in Guerro Provence, Mexico by F.R. Nick Nocerino. Sha-Na-Ra is still owned by Nocerino, while the Rainbow Skull is owned by DaEl Walker.
While many skulls were found near Mayan ruins, it is doubted whether the Mayans had the technical skill to create anything this complex. Instead, the Aztecs are pointed to as the creators, since they created more crystal artifacts than the Mayans, as well as having skulls featuring heavily in their art and religion.
More extreme theories have postulated the skulls to be artifacts of Atlantis or Lemuria.
However, there is growing certainty that most of these are modern fakes. Electronmicroscopy on the British and the Smithsonian skulls shows straight and regular markings on the skulls consistent with modern polishing wheels, which casts severe doubt on the 'sand and water' theory.