The most widely celebrated and mysterious crystal skull is the Mitchell-Hedges Skull, for at least two good reasons. First, it is very similar in form to an actual human skull, even featuring a fitted removable jawbone. Most known crystal skulls are of a more stylized structure, often with unrealistic features and teeth that are simply etched onto a single skull piece.
Second, it is impossible to say how the Mitchell-Hedges skull was constructed. From a technical standpoint, it appears to be an impossible object which today's most talented sculptors and engineers would be unable to duplicate.