Someone on this board once said that the Mitchell-Hedges skull was a "fake" that was found to have been made in the 1800s....this is false. It turns out the man's daughter probably didn't find it like he said; he *bought* the item in Mexico in the late 1800s.
The skulls themselves are impossible to date, because carbon-dating only works on organic material. Several of the skulls have strange properties; they were wrought against the grain of the quartz, which would normally cause the rock to splinter and shatter. Even today, modern technology cannot work around this. It's a basic property of crystals that you must cut them with the grain, yet the skulls, including the MH skull, are cut against the grain.
Analysis of the skulls shows none of the microscopic markings that would indicate they were "hand made," without the ultra-precise machinery we have today. This is one reason they are thought to be "fake" or modern by some. Yet, even dating back to the 1800s, which is the last documented acquisition of many skulls, the technology to create them so precisely did not yet exist.
They are indeed pretty mysterious artifacts...