Indiana Jones and the Jordan Connection
What's interesting here is that the late Anna Mitchell-Hedges claims to have found a crystal skull during her excavation of a 1924 Central American dig, and that without a doubt, this was the prime inspiration for the new Indiana Jones movie. Mitchell-Hedges was a Canadian orphan adopted by British adventurer Frederick Mitchell-Hedges in the early 1900s and died late in 2007 at the age of 100. A lot more can be found on the website below.
Suffice it to say that the Mitchell-Hedges story about finding the crystal skull, and all the material surrounding the crystal skull (whether it's real or not) would serve as an excellent starting point for a Pulp adventure rpg scenario, and could even recount attempts by dark forces to obtain the skull in question after the expedition.
And don't forget
The Official Indiana Jones website