CORRECTION: This story was updated on March 14, 2013, to indicate that the goblet pictured is pre-Columbian, not Aztec.

Humans seem to have a predilection for fake quartz-crystal Aztec skulls. Since the 1860s, dozens of skull sculptures have appeared on the art market purporting to be pre-Columbian artifacts from Mesoamerica, that is, created by the indigenous peoples of Mexico and Central America prior to Spanish exploration and conquest in the 16th century. Three such skulls have graced the collections of major museums on both sides of the Atlantic: the Smithsonian Institution in Washington, D.C., the British Museum in London, and the Quai Branly Museum in Paris.

FAKE DENTISTRY [+]Enlarge Credit: James Di Loreto/Smithsonian Institution GENUINE GOBLET [+]Enlarge Credit: Museum of Oaxacan Cultures SURFACE SECRETS [+]Enlarge Credit: British Museum GREEN EVIDENCE [+]Enlarge Credit: British Museum

As early as the 1930s, some experts began to have doubts about the authenticity of the skulls, says Margaret Sax, a conservation scientist at the British Museum. But for a long time researchers “didn’t have the scientific means to follow up” on their hunches, she adds. Over the past two decades researchers at all three museums have capitalized on analytical science innovations to show that these peculiar skulls are not unusual Aztec artifacts but post-Columbian fakes.

Nowadays the market for crystal skulls is limited to Indiana Jones fans, New Age devotees, and people in the goth and punk subcultures. But in the 1860s, when the skulls appeared on the market, many people in Europe sported little skeletons on rings, pendants, or other personal trinkets to remind them of their own mortality, says Jane Walsh, an archaeologist at the Smithsonian’s National Museum of Natural History. It was a French dealer named Eugène Boban who capitalized on this fascination with the macabre, as well as Europe’s growing interest in and ignorance of Mesoamerican artifacts, to slip some of the first sham skulls into museums.

Walsh has traced fake crystal skulls at the British Museum and the Quai Branly Museum back to Boban, who sold them to art dealers who then sold them to the museums more than 100 years ago. The Smithsonian skull, however, showed up in the mail in 1992, as an anonymous donation. Its arrival motivated Walsh to contact the British Museum to discuss the skulls. That conversation catalyzed the scientific and historical research that finally proved the objects were phonies.

The British and American team were particularly suspicious of the skulls because they hadn’t come from documented archaeological sites. And something was wrong with the skulls’ teeth. Although skulls do appear as motifs in Aztec art, most representations of teeth in authentic pieces reflect the dentistry—or lack thereof—of the time. The teeth in the suspect skulls seemed too linear, too perfect, Sax explains.

So the team took a closer look at the skulls’ surfaces. As a benchmark, they borrowed a legitimate Mesoamerican crystal goblet from the Museum of Oaxacan Cultures, in Mexico. Then they used scanning electron microscopy to compare these surfaces.

It turns out that the surface of the authentic goblet has irregular etch marks, a sign that the pieces were carved with hand-held tools. But the surface of the suspect skulls have regular etch marks, evidence that they were made with rotary wheels and hard abrasives, which appeared only after the Spanish conquest of Mexico, Walsh says.

Looking even closer at the British Museum’s skull, the team discovered green, wormlike inclusions in the rock. Raman spectroscopy revealed that the inclusions were an iron-rich chlorite mineral. Although this kind of trace impurity is found in rock crystal from Brazil or Madagascar, it is not found in Mexican crystal, Walsh says.

The team also noticed a small deposit of something curious in the Smithsonian’s skull. By using X-ray diffraction they discovered the deposit was silicon carbide, a synthetic abrasive used in stone-carving workshops only starting in the mid-20th century. This damning residue revealed the Smithsonian skull had likely been made mere decades before the anonymous donor sent the skull by mail, Walsh says.

As the British Museum and Smithsonian researchers began amassing evidence in the 1990s and 2000s that the skulls in their collections were certainly not of Aztec origin, museum staff at the Quai Branly Museum decided to scrutinize a crystal skull and a human head sculpture in their collection. Both objects were acquired through the controversial dealer Boban, and both were purported to be pre-Columbian.

The crystal skull at the Quai Branly Museum, like the fakes at the Smithsonian and British Museum, had a suspiciously perfect set of teeth, whereas the head had more realistic human features, the French researchers noted in a 2009 report (Appl. Phys. A, DOI: 10.1007/s00339-008-5018-9).

The French team dated the two objects using a noninvasive method that measures how deep water penetrates into rock objects. The method relies on shooting helium ions at an object’s surface and analyzing the interaction of the ion beam with the hydrogen in water at increasing depths in the sculpture. Then the water penetration is compared with samples of known ages. They found that the crystal skull had likely been made after the Spanish conquest, whereas the anthropomorphic head was likely made in pre-Columbian times.