In celebration of April’s birthstone, the diamond, we’re sharing an interesting new discovery about diamonds, courtesy of Michelle Graff and nationaljeweler.com.
In February, the Lucapa Diamond Co. in Perth, Western Australia announced that it has discovered the largest recorded diamond ever found in Angola: a 404.2-carat stone that has tested as Type IIa and D color.
The diamond was recovered from Alluvial Mining Block 8 at Angola’s Lulo Mine, which has produced more than 60 large, special diamonds since they started mining there just last August.
The company reported that the 404.2-carat stone is the 27th largest recorded diamond in the world, and the biggest diamond ever discovered by an Australian mining company. It also is the fourth 100-plus-carat diamond to be recovered from Lulo to date, as well as the 114th largest “special” diamond--meaning it weighs more than 10.8 carats--recovered from the mine.
Of further interest is the scientific work that geologists are doing on the unique properties of large Type IIa diamonds similar to the ones being found at Angola’s Lulo Mine.
Evan Smith, a postdoctoral research fellow at the Gemological Institute of America specializing in diamond geology, and his colleagues are trying to better understand Earth’s mantle, which is beneath tectonic plates and, as such, largely inaccessible for scientific observation.
As you may already know, Type IIa diamonds have very little to no nitrogen, which is what makes white diamonds so exceptionally colorless and fancy color diamonds so vibrant. Large Type IIa diamonds that make headlines also tend to be irregular in shape, rather than the nice, symmetrical octahedrons like so many smaller stones. They often have a surface that’s rounded and somewhat dissolved, “almost like a lollipop after someone’s been after it for a while,” says Smith.
The fact that these big, beautiful diamonds are different has not escaped the attention of earth scientists, who have wondered for years if they form in a different way, in a different part of Earth’s mantle, and thus tell us something different about our planet.
In order to conduct the study, though, Smith and the other researchers could not limit themselves to these kinds of large and exceedingly rare diamonds. Instead, they studied Type IIa diamonds of all sizes that came through the GIA lab, including some that were smaller than a carat.
After examining 52 Type IIa stones (and one Type Iab) of all sizes at the GIA lab, Smith and the other researchers found that in nearly three-quarters of the diamonds (38 out of 53), the inclusions weren’t graphite but metallic, a solidified mixture of iron, nickel, carbon, and sulfur.
This is significant because it changes the way scientists think about how different elements, like carbon, nitrogen, and sulfur, are distributed. It also has broad implications for understanding the behavior of the deep Earth, including the recycling of surface rocks into the convecting mantle.
Smith said this discovery verifies what geologists have been theorizing for 10 or more years: that the Earth’s deeper mantle environment has a “light peppering” (up to 1 percent) of metallic iron.
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To read more about this study, read Michelle Graff’s interview with Evan Smith at nationaljeweler.com.