Various Native American legends speak of 13 crystal skulls with important information to impart/size of human skulls; these skulls being brought together are thought to be a great event in mankind so that we understand the past and the future. These skulls are carved from quartz crystals, unknown when they were carved; most are Mesoamerican (Aztec or Mayan).
The Mitchell-Hedges Skull
Most of the mystery surrounds the Mitchell-Hedges skull discovered in 1924 (Frederick Mitchell-Hedges claimed his daughter, Anne, found the skull in Mayan city Lubaantun; probably planted there), which is a heavy and perfectly carved natural quartz that is anatomically correct and looks like it was done with modern-day tools. No matter how, the process was clearly sophisticated; even by more modern standards of creation, its conception is a mystery since quartz has the tendency to shatter and no evidence of modern tools on the skull (so it was made by hand). It is said to have psychic amplifier abilities/transmits information through the quartz crystal. It is unknown where or even when (since quartz does not age) it comes from, though the crystal is similar to Californian quartz. It is thought to have been rescued before the destruction of Atlantis or that they hail from extraterrestrials with much more sophisticated tools.
The skull can induce trances, can make some go insane in its presence, strange smells/mysterious sounds have emitted from it supposedly, aura emitting from it, seeing images in it of past/future, can induce fires if the light is just right, was likely used for healing (and apparently has healed deadly diseases).
Knowledge is imparted from the Mitchell-Hedges skull through channeling and it has said that basically inter-dimentional beings created the skull; it is a tulpa (also known as a thought-form), channelled into being by thought alone. It tells us explore the oceans because evidence of this previous culture is there and that the separation humans crave is the source of all problems in our world and our messing with our world through radiation/atoms is causing all the weather/climate/axis issues.
New Age Crystal Skulls
Crystal skulls could also be connected to New Age ideas of starseeds/lightworkers/Lemurians (which are either people sent to Earth to help with healing; Lemurians are an ancient race of higher-consciousness beings).
Erich Von Däniken
One thing that has to be said: ancient non-white people created amazing things and it’s racist to assume they didn’t or that aliens helped them instead. An example of this is Erich Von Däniken assuming the Nazca Lines in Peru were extraterrestrial in nature because ancient non-white people couldn’t have accomplished such a thing: it has to be aliens. (Its been proved the Nazca Lines can be made naturally, by the way.)
Judith Joyce. 2010. The Weiser Field Guide to the Paranormal: Abductions, Apparitions, ESP, Synchornicity, and More Unexplained Phenomena from Other Realms.
Chris Morton and Ceri Louise Thomas. 2012. The Mystery of the Crystal Skulls.