Tiger chert is a stone that formed about 50 million years ago in Wyoming and the surrounding region, and artifacts made from the rock are found around the West. It's made of quartz and has distinctive stripes that give it its name.
"There are many, many different sources of stone for making stone tools in prehistory, but tiger chert is really distinctive in its appearance," said University of Wyoming anthropology professor Todd Surovell. "It's most commonly found in artifact form in southwest Wyoming, but it's also been shown archeologically to have traveled huge distances."
Not many types of stones travel that far.
"For example, there are several artifacts made of tiger chert that have been found as far south as Arizona and southwest Colorado, down in the Pueblo world," he said. "Almost certainly this material was being traded long distances because it's very unique and distinctive in appearance."
According to Surovell, prehistoric people placed a high value on the rock and it was a commodity.
"What's really interesting is in prehistory, there were several materials from Wyoming that were widely traded commodities, tiger chert being just one of those," he said. "If you look at the modern economy of Wyoming today, we have examples of very similar economic processes happening going back thousands of years in prehistory."
Normally, archaeologists will space themselves out and walk across a site in a grid so they don't miss anything. That's how they find any artifacts, including those created with tiger chert. But a new technique using infrared images might make things easier.
"Fluorescence happens when you shine light on something and it gets absorbed, but it doesn't get completely absorbed," U.S. Air Force Academy chemistry researcher Campbell Andersen said. "Some of that light gets reflected at a lower wavelength with less energy."
Tiger chert fluoresces. Andersen said that phenomenon isn't limited to rocks.
"The most common fluorescence you're probably familiar with is going to be fluorescence like a bowling alley," he said. "You go in there and they've got these blacklights, and then your white shirt is like super white because it's fluorescing under the blacklights."
Andersen said when they shine blue light on tiger chert, it re-emits it in the infrared, just past the colors that humans can see. But digital cameras can pick it up and it helps researchers find the artifacts.
"Using that phenomenon, if you shine blue lights on a big swath of land out there where you've found this stuff before and then take a picture of it with your infrared camera, you can see 'oh there's one, there's one, and there's one over there' that you might not have been able to see just using your eyes," he said.
Andersen said there aren't a lot of rocks that fluoresce in the infrared, so this is a useful way to find tiger chert.
But according to Surovell, the stone is so distinctive that special methods to find it like this might be overkill. But the technique might be helpful in other ways.
"In cases where you have stone that is moved up to five, six hundred miles, huge distances, and you want to make a claim that you've identified the source for that material, using a more scientific or more analytical approach to identify it definitely strengthens your conclusions in that identification," he said.
Surovell said the concept of using infrared images to find artifacts in the field needs to be tested, and he's part of an effort to do so.
"I'm on a research project right now that's NASA funded," he said. "We are trying to use infrared cameras on drones to automate the mapping of archaeological sites using distinctive mineralogical characteristics of archaeological materials on the surface."
This is challenging because according to Surovell, most artifacts and the sediment around them are made of similar material. But by using special characteristics, like those discovered by Andersen's team in tiger chert, they may be able to map archaeological sites better and faster than ever before.
