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“Our job is not just to measure the isotope composition of the inclusion, but also to get the whole picture of how the diamond formed, before we end up destroying the diamond to get the inclusion out.” The first step is to slice the diamond very precisely across its growth zones using a diamond-cutting laser.
“It would probably drive everybody in the gem business crazy to understand that we take a beautiful rough diamond and slice the center out of it,” says Shirey with a wry smile.
The process begins with suitable inclusion-bearing diamond crystals.
As Shirey tells us, obtaining such diamonds for study is quite a challenge.
“We have to get the inclusion out without breaking it,” says Shirey.
“We need to recover the whole inclusion, and we also need to characterize the diamond as fully as possible.
Research using the rhenium-osmium decay system proves that some diamonds are of remarkable antiquity, says Shirey.
“They’re sometimes the oldest minerals we can find on the earth…up to 3.5 billion years old, whereas the earth is only 4.5 billion years old, so they’re often three-quarters of the age of the earth.” Shirey adds that diamonds are also special because they’re the deepest minerals we can obtain as natural samples to study the earth.
The isotope of rhenium he uses, years, or 41 billion years.
These “superdeep” diamonds provide Shirey and other scientists a tantalizing window into the workings of the deeper mantle.
Peter Johnston © GIA paper “Recent Advances in Understanding the Geology of Diamond,” coauthored by Shirey and GIA’s Dr. Shirey is a one of a collaborative group of geoscientists from institutions all over the world using diamonds as a means to sample the deep earth.
“We have to go to a mine or some place that’s being very aggressively prospected so that they’re processing large amounts of kimberlite for diamond grade.” Diamonds are trace minerals in the rock—kimberlite, or more rarely a lamproite as in Australia’s Argyle mine—that carried them up from the mantle.
“A diamond in a kimberlite occurs at the part-per-billion level,” says Shirey, “so the average person walking around on a kimberlite is not going to find a diamond sitting there—that’s an extremely rare occurrence.” Once researchers have traveled to suitable mining or exploration operations, where large amounts of diamond-bearing ore are produced, they have to pick through the production.