Determining the true origin of the “Hope Diamond” and the “Kohinoor”

Scientists have revealed that they may have finally found the true origin of the Hope Diamond, the Kohinoor Diamond and other “flawless” gemstones.

These diamonds, collectively known as Golconda diamonds, are special because they contain few inclusions and are very low in nitrogen, making them extremely clear and free of blemishes that mask the sparkle. Plus it's big.

The Kohinoor is famous for being one of the British Crown Jewels, and it weighs 105.60 carats. While the Hope Diamond, located in the Smithsonian Institution's National Museum of Natural History in Washington, DC, weighs 45.52 carats.

These diamonds were discovered in southern India between the seventeenth and nineteenth centuries and carry amazing stories.

Most of these distinctive gems are now held outside India, and there are calls to repatriate many of them due to their cultural and religious significance.

Golconda diamonds were found in so-called alluvial mines, which are shallow pits in riverbank sediments. The diamonds were transported with these deposits to the banks of the river. But diamonds come to the Earth's surface through a large volcanic eruption called kimberlite, and no one knew where to find the kimberlite rocks that hold these diamonds.

Now, new research published in the journal Earth System Science suggests that the diamonds may have come from the Wagrakarur kimberlite field in modern-day Andhra Pradesh, up to 300 km (186 miles) away from where they were mined.

Scientists studied the geochemistry of common diamonds from the lithosphere (the solid crust and upper mantle), and suggested that the Wagrakarur field could host diamonds. However, Golconda diamonds form deeper in the mantle, perhaps as deep as the transition zone near the Earth's core.

Yakov Weiss, a geochemist who studies diamonds at the Hebrew University of Jerusalem, and who reviewed the paper prepared for publication, explained to Live Science: “The analysis is mainly related to diamonds found in the lithosphere, and we believe that larger diamonds come from deep within the Earth.” “So there is still some uncertainty.”

In an attempt to trace the source of the Golconda diamonds, Hiro Kalra, Ashish Dongre, and Swapnil Vyas, all geologists at Savitribai Phule Pune University in India, studied the chemical signatures of nearby kimberlites and lamproites. These are rocks that came from the base of the crust and upper mantle, where most diamonds are formed.

They found that kimberlite rocks from the Wagrakarur field likely rose from depths, where diamonds are formed and host minerals that tend to coexist with diamonds. They then conducted surveys using remote sensing data, such as satellite images and measurements of vegetation and humidity.

These surveys revealed an ancient, long-drained river channel that may have transported diamonds from Wagrakarur to the Krishna River and its tributaries, where the distinctive gemstones were eventually found.

Weiss cautioned that linking the kimberlite field, where standard lithospheric diamonds are found, with deeper Golconda diamonds, is not easy.

This deeper diamond has a different chemistry, and could theoretically have come from somewhere else. 

No one knows exactly how these deep diamonds reach the surface of the Earth. They may rise from the deep mantle on fountains of hot magma known as mantle plumes and then become trapped in the lower crust and upper mantle, with more ordinary diamonds forming in those regions. Then, when a kimberlite eruption occurs (possibly as a result of the breakup of a supercontinent), all the diamonds move to the surface at once.

However, it is very difficult to detect the origins of Golconda diamonds directly, because these diamonds lack the small inclusions that carry fluids from the mantle where the diamonds first formed.

This makes them beautiful and desirable as gemstones, but it gives geochemists very little information to work with, Weiss said. As a result, Golconda diamonds will likely always retain some mystery.