It's Just A Pebble: That's what a South African farm wife told amateur geologist Schalk van Niekerk in 1866 when he took interest in a rock that her children were playing with. Niekerk offered to pay for it, but she told him to take it. So he did—to all the experts he could find in the backwater colony of the Cape of Good Hope.
It's A Diamond: Or so Cape Colony's foremost geologist, William Atherstone, announced in 1867. By his calculations, the "stone" was actually a 21-carat gem worth about Â£500.
No, It's A Pebble: You'd think van Niekerk's little gem would have sparked a South African diamond rush, but instead, the discovery just made people very, very skeptical. At the time, South Africa's geography was thought to be unfit for producing diamonds, and no one was interested in changing their opinion. Instead, van Niekerk's diamond was widely considered a scam—an attempt by yokel colonists to pull one over the eyes of wealthy, would-be investors. In fact, the Illustrated London News refused to publish an image of the stone, lest it tempt the naive. Even the jeweler who positively ID'd the diamond refused to send people out to look for more.
No, It's a Diamond!: It's worth noting that, while all this naysaying was going on, many more diamonds were being found in the area.
Diamond!: Unfortunately for Gregory, shortly after his investigation, a massive 83-carat diamond was found in the very area he'd declared diamond-free. The gem became known as the Star of South Africa and launched the region's first mining investments. To this day, diamond industry honchos still refer to bad judgments or misstatements as "pulling a Gregory."
This summer, we'll be re-running parts of "The 20 Greatest Mistaikes in History," Maggie Koerth-Baker's cover story from March-April 2007. For other installments, click here.