Seeking the verdict of science, researchers from those two museums examined the skulls with electron microscopes, looking at tiny scratches and marks left by the carving implements.
These were then compared with the surfaces of a crystal goblet, rock crystal beads and dozens of greenstone jewels known to be of genuine Aztec or Mixtec origin.
The study appears in the Journal of Archaeological Science, published by the Elsevier group.
I liked this part: The investigators also found a black-and-red deposit in a tiny cavity of the Smithsonian skull. X-ray diffraction showed it to be silicon carbide -- a tough compound that only exists naturally in meteorites but is widespread in modern industrial abrasives.
I linked to a couple of sites showing that most believe the skulls were fake modern anyway, based on lack of provenance, etc. So, no big news amongst the archaeoliterati.