There are many more than thirteen crystal skulls across the world these days. However, there are thirteen notable skulls that are made of different kinds of crystal. The first complete and most famous is the Mitchell-Hedges skull that was found in an ancient Mayan temple. It's cut of only one piece of crystal and it's cut in a way that scientists are saying it should have split into a billion pieces... but it didn't. The lower jaw can be removed and everything. There is absolutely no traces of carving or really any marking at all on these things... it's amazing. The only thing that could cut through the crystal the Mitchell-Hedges skull was cut of is diamond... and the man-power it would take back then would add up to about three hundred year's worth of work. Also, the size and type of crystal wasn't available to that area... so maybe the crystal came from somewhere else or maybe some other civilization made the skull. Aztecs had an obsession with skulls. Who knows.
No one's got any idea where they came from. It's all guesswork. Most of them have been found in South America but there have been others. They've been juggled around and traded and sold and auctioned. Finding where they came from is almost impossible. Just like making those skulls were. They sent the Mitchell-Hedges skull to various crystal experts and crystal optical equipment manufacturers for examination. They said it was cut against the natural axis of the crystal, meaning that it SHOULD HAVE split. And yet it didn't... some sources I've looked at have said that it is impossible for us with even our lasers and technology to duplicate it.
No one's sure where the hell they came from. The Mayans thought they were from the Goddess of Death. But no one can be sure... there just isn't any trace or proof of them. They supposedly can induce dreams of the Mayan civilizations and rituals. In one case there was a skull that was on display in a museum. The workers demanded they cover it up at night because they said that it moved around on its own.
They're also not sure why they exist or how old they are. They have all these prisms and such inside that create different light patterns and optical illusions when subjected to light and color patterns. It's strange. Some people think that the skulls have something to do with the date of December 21, 2012 which is the end of the Mayan calender. But then again, not all the skulls are of Mayan origin though most of them suggest it. Who knows.
That's my bit. If you want to read the full thing here's the site.
http://www.mendhak.com/paranormal/mystery/show.php?id=25