As far as natural phenomena go, blue diamonds are among the world's most treasured. Known as Type IIb diamonds, their fame has only been boosted by the most well-known example, the Hope Diamond. A new study examining their rare properties suggests that their signature blue come from an equally rare source: the Earth's lower mantle.

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Type IIb diamonds form when carbon atoms form intensely strong bond. Their greyish-blue features come from trace particles of boron, comparatively less rare than diamonds, caught up within these bonds. These diamonds formed around a billion years ago, during the Earth's formative years.

Scientists were never quite sure how the boron got there, but the new study from the Gemological Institute of America tells a very specific story of creation using mineral inclusions. Inclusions are small pieces of material trapped in the diamond during crystallization, and the researchers found that these inclusions were found together at extremely high pressures at unusually deep depths.

The inclusions showed that blue diamonds formed anywhere between the Earth's transition zone all the way to the lower mantle. That means these diamonds were forming anywhere between 250 to 1800 miles below the Earth's surface. That's four times the depth of most diamonds.

At this depth, water-bearing minerals that fell down from the Earth's surface were broken down through sheer pressure and temperature. Once broken, the minerals release a boron-enriched liquid that triggered the creation of diamonds which cost millions today. The study's findings have been publishing in Nature.

Diamonds feel like a natural rarity, but that's not necessarily the case. Last month, scientists showed that there are likely quadrillions of diamonds below the Earth's surface. The problem? Like blue diamonds when they formed, they're buried too deep for humans to reach them.

Source: Gemological Institute of America