Dramatic stone masks, iconic finds in the ancient Mexican city of Teotihuacan, were supposed to be made from a jadelike stone. Many researchers also thought the large faces were made on the site of the pre-Columbian metropolis. Instead, they seem to have been made in workshops a great distance to the south of the city. And they are made of softer stone like serpentinite and polished with quartz. Quartz does not appear around Teotihuacan, bolstering the notion that the masks were made far away. “Almost everything that has been written about the making of the Teotihuacan masks is untrue,” says Jane Walsh, an anthropologist at the Smithsonian National Museum of Natural History in Washington, D.C.

New details about the manufacture of these old and valuable masks are coming to light, thanks to modern technology: a special analytical scanning electron microscope that can identify the atoms and minerals that make up the stone, and show miniscule marks left by the artisans who carved them. Timothy Rose, a geologist at the Smithsonian, presented the results of microscope studies last week at the annual meeting of the American Vacuum Society, a group of material scientists, in Baltimore. “We examined about 150 of these masks with good provenance, from several museum collections,” says Rose, who works with Walsh.

Within that group he found about three fakes, uncovered because the microscope showed the telltale marks of modern tools. “We were pleased the number was so small,” he adds. “But it is important to have this technology to tell what is real, because many masks, in both museum and private collections, don’t come with good documentation about their origins.”

There is plenty of other evidence that Teotihuacan had far-flung connections, notes Geoffrey Braswell, an anthropologist at the University of California, San Diego, who studies Mesoamerica. Other imports have been found in the city, he said in an email. Walsh and Rose, however, note that the mask-makers have not been traced.

At its peak of power, in the first half of the first millennium A.D., Teotihuacan, which is just north of present-day Mexico City, was the largest city in the New World. It had with more than 100,000 inhabitants, tall pyramids, multistory apartment buildings, broad avenues and ceremonial structures. The society dominated even the Maya of that time who lived to the east in what is now southern Mexico and Guatemala. The city collapsed, for unknown reasons, in the seventh century A.D. and its buildings were sacked.

More than 600 of the stone masks now are held in museum collections throughout the world. Walsh says that researchers thought they were made of jadeite, a hard rock. But no one had really analyzed their composition.

Rose did, using a special scanning electron microscope. Usually such microscopes require samples to be coated with particles like carbon or gold in order to produce images. That cannot be done with valuable archaeological artifacts because it would damage them. But Rose uses a low-vacuum microscope, which surrounds the specimen with water vapor instead. The microscope’s beam hits electrons in the vapor, which interact with electrons on the sample surface to produce a detailed image. It also produces a signature from the atoms in the sample. “We can tell what kind of atoms are in any material, and therefore identify any minerals and rocks,” he says.

The masks turned out to be made of minerals such as serpentinite, limestone and travertine rather than jadeite. The scientists also identified harder grains of quartz on the surface. Quartz makes a good abrasive for stone polishing. It is not found near Teotihuacan but it does appear about 150 kilometers south in the state of Puebla. In Puebla, archaeologists have found remains of workshops used to make ceramic objects that are characteristic of Teotihuacan. “We think the masks were manufactured by artisans in Puebla, carved and polished there, and then brought a long distance to the city,” Walsh says.

More support for this interpretation comes from other material found on the masks: Skeletons of diatoms, a kind of algae. “They sort of look like hair rollers,” Rose says. These skeletons form diatomaceous earth, a soft type of sediment that would be a good final polish for the stone masks. Again, such earth is not found near Teotihuacan but does appear in Puebla.

Because the scanning electron microscope reveals fine details, as small as one micron in diameter, the scientists were able to see the tool marks of the mask makers. (A micron is one millionth of a meter, or about 40 millionths of an inch.) That turns out to be a good way of telling old, real masks from modern fakes. “A pre-Columbian carver would use a stone file with sand to help it cut,” Walsh says. “What that leaves is a straight rut on the surface of the mask.” Modern forgers—spurred by the $150,00 to $250,000 such masks can fetch at auctions—use rotary cutters, which are much faster but leave a very different signature. “You can see curvature in the cuts,” she says. The modern blades are also studded with diamonds, which leave telltale parallel grooves.

It is reassuring, the researchers say, that most museum holdings appear to be genuine. And moving forward, as excavations at Teotihuacan continue and more masks surface, it is even more comforting to have a tool at hand that can spot the occasional frauds.