They are incredibly rare to find, due to the unlikely requirement of both a Restless Dead level feeling and a Golden Idol generating on the same level, but they are incredibly valuable when they do appear, trading for $30,000 if you can bring it to the level exit alive.
When the Crystal Skull is lifted from its platform, The Ghost appears immediately. Two Ghosts can be summoned at once using this, one from the time limit and one from the trap. However - just like other Idol traps - the platform that the skull rests on can be destroyed to evade the consequences. Picking up the Crystal Skull after destroying the platform it originally rested upon will not summon the ghost.
The area containing the Crystal Skull is apparently fully enclosed, but the two push blocks on either side are always loose, allowing access without bombs as long as you can jump up to push them. The Crystal Skull is also always guarded by four undead Skeletons in each corner of the treasure chamber.
The Crystal Skull, like the Golden Idol, is more homage to the Indiana Jones movies.