SciFri 4.1.05: Land of snow and ice?

The world's largest cut diamond, the Cullinan I, also known as the Star of Africa.

Photo courtesy of famousdiamonds.tripod.com

Minnesota is ripe for diamond exploration

By Deane Morrison

Published on April 1, 2005

The tiny rock weighs only seven-thousandths of an ounce, but people gladly pay upwards of $1,000 for it. It's a one-carat diamond, a gem most of us associate with southern African mines. But diamonds are also found much closer to home. In the past decade, Canada has become the supplier of 10 percent of the world's diamonds, including stones that equal the best gems in the world. Minnesota, which has the same geology as the diamond-producing areas of Canada, could be poised to enter the market.

Diamonds usually form deep in the earth, in areas where rocks are at least 2.5 billion years old and 120 miles thick. (There are exceptions, including diamonds from Arkansas and the prized pink diamonds from Australia, which originate in younger, thinner rocks.) Such depths are necessary to generate the tremendous pressure and temperatures exceeding 3,500 degrees F that cause carbon to form crystals with the characteristic diamond shape. Relicts from a very early stage in the planet's history, diamonds are not only the hardest stones but extremely heat-resistant, melting near 7,200 degrees F--nearly two-and-a-half times the melting point of steel. Their hardiness makes diamonds indispensable components of drills, cutting tools, and electronics.

Depending on what impurities the stone contains, diamonds come in many colors: yellow, brown, pink, red, blue, purple, green, black, gray, violet and orange. Yellow and brown are most common, and red and green the rarest.

The carbon that forms diamonds is the same element as the graphite in your "lead" pencil. It also forms the backbone of organic matter. In fact, there is evidence that the carbon in some diamonds comes from the remains of organisms, probably oceanic plankton whose remains were carried deep into the earth when continents collided. And there they would have stayed, miles below the surface and the notice of humans, were it not for occasional upbursts of magma, Earth's molten core.

"Magma sometimes pops like champagne through a fissure and breaks through rocks above it," says Harvey Thorleifson, director of the Minnesota Geological Survey and an expert on diamond hunting. As the magma shoots up toward the surface, it expands slightly and picks up rock in its path, including diamonds. When it cools, it forms a vertical cone-shaped chimney of rock, called kimberlite after the Kimberley diamond mine in South Africa.

"Kimberlite is broken rock," says Thorleifson. "It's a hodgepodge of the magma and whatever rock was carried along with it." Kimberlite chimneys are typically small, less than half a mile across at the surface.

As the kimberlite weathers, it may release its cargo of diamonds, which get swept away by rain and end up in riverbeds or deltas. Many diamonds have been found in stream sediment, and rich sources of the gems have been found by tracing the trail of diamonds up-river. Another way to find diamonds, says Thorleifson, is to follow the trails of "indicator minerals" like chrome diopside, a lovely emerald-colored mineral found in kimberlite. Indicator minerals are much more abundant than diamonds, and following the trail toward ever higher concentrations of the minerals can lead to the common source of both indicator minerals and diamonds. Another method, which has proved useful in North America, is to trace trails of diamonds back toward the sources of the glaciers that carried sediments south from Canada. That won't, however, lead to lodes of American gems.

If Minnesota has any kimberlite chimneys, they must lie under the state's thick blanket of glacial deposits. To find them, diamond hunters will need sophisticated means such as magnetic mapping. In his office, Thorleifson has a map of magnetic readings from rocks all over the state. Here and there are tiny round areas that look like pinpricks on the map; these, he explains, are magnetic anomalies, where nearby rocks tweak the compass needle. Some of them may be the round tops of kimberlite chimneys.

If you're lucky enough to find what looks like a diamond in the rough, the key to identification is right on the surface of the stone, Thorleifson says.

"With a magnifying glass, you can see triangular patterns called 'trigons' etched on the surface," he says. "They are defects on the surface of the crystal."

A good diamond, especially a colorless one, reflects and refracts light from its many facets, and this is what makes it beautiful. Depending on what impurities the stone contains, diamonds come in many colors: yellow, brown, pink, red, blue, purple, green, black, gray, violet and orange. Yellow and brown are most common, and red and green the rarest. Diamonds are judged on the four C's: cut, color, clarity and carat (weight), a carat being one-fifth of a gram.

The 3,106-carat Cullinan diamond from South Africa's Premier mine is the largest diamond gemstone ever found. Discovered in 1905, the stone was cut in Amesterdam. The largest gem from the cutting, pear-shaped, 530-carat Cullinan I, now resides in the head of the royal scepter of the British crown jewels. It is the world's largest cut diamond. The second-largest gem from the same stone, the 317-carat, cushion-shaped Cullinan II, was set in the British imperial state crown.

Minnesota is unlikely to turn up any gems like the Cullinan. But if Canada's experience is any indication, a diamond industry could be profitable. Thorleifson, who worked for many years with the diamond industry as a scientist with the Canadian government, says exploration expenditures in Canada total $100 million (U.S. dollars). The result has been a $1 billion per year industry. Not bad for a bunch of carbon crystals.