Not to disapoint, but it was proven to be a hoax
The original claim to fame of the skulls was that the sophistication of the carvings from the particular time period was incredable. Then someone interpreted that to mean they weren't made by man but by some alien race. Which lead to an increase to their popularity. The latest Indiana Jones movie plays this up a bit as well.
Unfortunately, what was truely taking place was that the archeologists that were working on the mayan ruins were finding that their site wasn't having enough of a pay out for artifacts and their grants were dwindling. The skulls were brought over from europe and the find was staged.
Tests on the skulls later show that the crystal used isn't of Brazil or even South American origins but from Europe (may have been carved in germany) and the tooling indicates mid-19th century manufacturing.
http://en.wikipedia.org/wiki/Crystal_SkullBut for those who still believe here is more info on the skulls found
http://www.world-mysteries.com/sar_6_1.htm