I found the 13 Crystal Skulls attunment interesting because the special, sacred energy that can be used for the good of mankind and its healing properties mostly
become heightened when the number of crystal skulls form a circle.
It is intensified when the twelve skulls form the circle and the 13th is located in the middle. It seems to me that all the energy from the circle is focused at the Skull, either as a master or the Scape Goat, the 13th location, with all the 12th Skull energies. focussed upon and as a healing source.