During the summer of 1872, two prospectors, John Slack and Philip Arnold, began flaunting a sack of diamonds and other sparkling gems to wealthy investors in San Francisco. They claimed to have discovered a massive diamond strike on their secret mining claim somewhere in the Western U.S. Fearful of missing out on the next big bonanza, the city’s elite quickly pledged millions of dollars to share in the opportunity.

Except this diamond field didn’t actually exist.

Fortunately, a chance meeting between a financier’s agent and a geologist named Clarence King stopped the shysters in their tracks. After hearing of the claim and piecing together clues to its location, King, a federal employee who was conducting a geologic survey across the 40th parallel—just south of the Wyoming border—rushed to the site.

Once there, King and his team did indeed discover diamonds, but their initial excitement was soon tempered by some unsettling realizations. Each diamond, they observed, was accompanied by 12 rubies—no more, no less. How could Mother Nature be so perfect? Next, the geologists discovered other gems such as amethysts, garnets, and spinels alongside the diamonds. These jewels form in very different conditions than diamonds and had never been know to coexist.

Last but not least, the party found a cut diamond—conclusive evidence that the gems had been “planted” at the site. King rushed to San Francisco to warn the financiers that they had been duped, narrowly averting a major economic crisis.

The story unexpectedly resumed in 1964 when a Colorado State University professor named Malcolm McCallum discovered a small pipe of an extremely rare type of volcanic rock in northern Colorado. Because this rock erupts from incredibly deep depths—up to 90 miles—in the Earth in less than a day, it is the only known material that can transport diamonds, which form deep in the Earth’s mantle, to the surface along with it.

McCallum’s finding set off a flurry of exploration. Geologists quickly realized that a narrow line of these unusual volcanic rocks stretches from Boulder all the way to Wyoming’s Laramie Mountains and that some of them actually do host diamonds—enough, in fact, for Colorado to become the site of the continent’s first commercial diamond mine, which operated until 2003. All told, the field yielded more than 130,000 diamonds, including a gem-quality, 28.3-carat yellow specimen.

In the case of the Great Diamond Hoax of 1872, Mother Nature, it seems, got the last laugh.

Terri Cook
Terri Cook
Terri Cook is an award-winning freelance writer based in Boulder. More of her work can be found at down2earthscience.com.