A bit more of the wiki article Miniatus touched on above is pretty conclusively damning to the provenance being claimed by some.
The crystal skulls are a number of human skull hardstone carvings made of clear or milky quartz rock, known in art history as "rock crystal", claimed to be pre-Columbian Mesoamerican artifacts by their alleged finders.
However, none of the specimens made available for scientific study have been authenticated as pre-Columbian in origin. The results of these studies demonstrated that those examined were manufactured in the mid-19th century or later, almost certainly in Europe.
Despite some claims presented in an assortment of popularizing literature, legends of crystal skulls with mystical powers do not figure in genuine Mesoamerican or other Native American mythologies and spiritual accounts
and furthermore, the wiki writeup goes on to say..
Research carried out on several crystal skulls at the British Museum in 1967, 1996 and again in 2004 has shown that the indented lines marking the teeth (for these skulls had no separate jawbone, unlike the Mitchell-Hedges skull) were carved using jeweler's equipment (rotary tools) developed in the 19th century, making a supposed pre-Columbian origin problematic.
The type of crystal was determined by examination of chlorite inclusions, and is only to be found in Madagascar and Brazil, and thus unobtainable or unknown within pre-Columbian Mesoamerica.
The study concluded that the skulls were crafted in the 19th century in Germany, quite likely at workshops in the town of Idar-Oberstein renowned for crafting objects made from imported Brazilian quartz at this period in the late 19th century...
A detailed study of the British Museum and Smithsonian crystal skulls was accepted for publication by the Journal of Archaeological Science in May 2008.Using electron microscopy and X-ray crystallography, a team of British and American researchers found that the British Museum skull was worked with a harsh abrasive substance such as corundum or diamond, and shaped using a rotary disc tool made from some suitable metal.
The Smithsonian specimen had been worked with a different abrasive, namely the silicon-carbon compound carborundum which is a synthetic substance manufactured using modern industrial techniques. Since the synthesis of carborundum dates only to the 1890s and its wider availability to the 20th century, the researchers concluded "[t]he suggestion is that it was made in the 1950s or later".
One archaeologist in particular had a rather interesting perspective regarding the crystal skulls being displayed as museum exhibits..
Crystal skulls have been described as "A fascinating example of artifacts that have made their way into museums with no scientific evidence to prove their rumored pre-Columbian origins
I think this one is pretty clearly a fun story that has taken on a life of its own over the course of a few generations.