Nature’s Gift to Toolmakers: Earth’s ‘Sharpest’ Substance

YELLOWSTONE NATIONAL PARK, Wyoming — Near the north entrance to Yellowstone National Park, an imposing mountain of black glass stands against the blue sky.

Spanning more than 13 square kilometers, the dark, sometimes translucent mass was formed from a rhyolitic lava flow that oozed out of the magma chamber of Yellowstone Caldera beneath the park, and cooled rapidly in the bitter cold of a glacial maximum, about 180,000 years ago.

Known as Obsidian Cliff, the mountain is one of America’s highest quality deposits of “the sharpest natural substance on Earth,” according to Douglas H. MacDonald, a professor of anthropology at the University of Montana.

Obsidian is among the most prized tool stones in the world. The smooth, sensuous black glass has been an important part of hunter-gatherer cultures for about two million years, going back to Homo habilis, one of the first human ancestors who made stone tools.

The Obsidian Cliff deposit, nearly 30 meters thick, is exceptional because of its continual use by Indigenous people since the last ice age. Over the last 11,500 years or so, the stone has been fashioned into knives, razor-sharp spear points, darts for atlatls, or spear-throwers, and arrowheads.

The cliff is “nationally significant because we had Native American groups from all over the country visiting it and collecting the stone and trading for it,” Dr. MacDonald said.

For researchers, the obsidian columns of Yellowstone have helped to reveal the travels and migration of people thousands of years ago. X-ray fluorescence technology has been used to identify the unique geochemical fingerprint of each separate deposit of obsidian, pinpointing the provenance of artifacts found elsewhere. Obsidian from here has been found across the continent.

The technology opens up unknown and unimagined connections and deepens the understanding of migration, networking and trade in populations around the world.

“We can figure out where people are moving on the landscape and from there how the tools themselves reflect their strategies and culture,” said Elizabeth A. Horton, the archaeologist for Yellowstone, as she displayed obsidian knives and spear points from the park’s archives.

The application of X-ray technology to archaeology arose in the 1960s “and changed everything,” said M. Steven Shackley, director of the Geoarchaeological XRF laboratory in Albuquerque, New Mexico. “Before that you had to infer.”

One lingering mystery of Obsidian Cliff stumps scientists to this day: how, some 2,000 years ago, hundreds of kilograms of obsidian wound up at what is now Hopewell Culture National Historical Park in Ohio, about 2,700 kilometers east of Yellowstone. Researchers do not know if the material traveled through trading networks or was gathered by people who went on a journey to the cliff.

“If someone sends me an artifact, I can determine the origin of it with a good degree of confidence,” Dr. Shackley said. “But determining how it got there is the kicker.”

In the early 1890s, excavators digging in Ohio found an altar with more than 100 burned and broken obsidian spear points, also known as bifaces. About three-fourths of the obsidian at Hopewell has been traced to Obsidian Cliff using the X-ray tech.

“The largest of these bifaces are master works, really remarkable,” said Timothy D. Everhart, a museum curator and archaeologist at the Hopewell park.

All obsidian shares certain critical features that made it indispensable, the main draw being sharpness. “If you look at a surgical scalpel and a fresh obsidian flake under a microscope, the obsidian edge makes the surgical scalpel look like a dull ax,” said Ellery Frahm, an archaeological scientist at Yale University. In fact, some surgeons use obsidian scalpels.

Obsidian objects have long been imbued with profound spiritual and mystical properties of other millenniums. The name of the Aztec god Tezcatlipoca was Lord of the Smoking Mirror, a reference to obsidian. An Aztec mirror displayed at the British Museum was used in the 16th century by John Dee, an adviser to Queen Elizabeth I, as a tool for divination. X-ray fluorescence tracked the source of the “spirit mirror” obsidian to Mexico to reveal its Aztec origins.

Obsidian Cliff, which is designated a National Historic Landmark, still figures in the lives of many tribes.

“Anytime we go to Yellowstone, that’s a really powerful place,” said Louise E. Dixey, cultural resources director for the Shoshone-Bannock tribes in Idaho. “Prayers are always said there. We go back every year to remind our people where we came from and remind the non-Indian public that these are our original homelands.”