"A report from De Beers’s new diamond mine" in The Economist included this bit of history.
"Speculation that diamonds might be found in Canada dates from the 19th century, when gems were found studded through the American Midwest. In 1888, the year Cecil Rhodes founded De Beers in South Africa, a 22-carat stone was unearthed near Milwaukee. Glaciers, it was posited in 1899, might have carried the diamonds south. It was decades before exploration took off. De Beers began quietly scouring Canada in the 1960s, but it was not until 1991 that BHP, one of its rivals, found kimberlite, an igneous rock, with enough diamonds to merit a mine."That local find was news to me, and led me to a quick search as a result of which I learned of the Eagle Diamond.
There's a bit more on it at the Wisconsin Historical Society.