We Finally Know Where Extremely Rare Blue Diamonds Come From

The Hope Diamond, a particularly storied example. 350z33/Wikimedia Commons; CC BY-SA 3.0

Diamonds aren’t always pristine. In fact, most of the time they contain inclusions – trapped chemical compounds they’ve picked up along the way – which can give them a welcome or unwanted hue. Take blue diamonds, for example: although rare, it’s well known that it’s the element boron that gives them their famous shade.

As explained by a new Nature study, precisely where this boron comes from has been something of a mystery for some time now. Fortunately, after tinkering with the geochemistry of dozens of these blue diamonds, the Gemological Institute of America (GIA) has provided us with an answer.

Boron is near-ubiquitous in the crust, both oceanic and continental. Clearly, this is getting sucked down deep below to depths where diamond-forming fluid accumulates, but it’s not been clear how it got there.

Taking a closer look at the mineral inclusions in these diamonds – many of which hadn’t been seen before – using X-rays and lasers, the team matched them with mineral ancestors, geochemical precursors if you like, found in oceanic crust segments.

Specifically, these were segments that trapped seawater and dragged it into the mantle – the superheated, colossal, solid layer that hides beneath the crust. As these broken crustal giants headed through this layer, the seawater-enriched rocks were “cooked”, and boron-rich fluids were released.

The physical structure of these mineral inclusions also betray just how deep these blue diamonds form: in the lowest parts of the mantle, at depths of at least 660 kilometers (410 miles). According to the study, this makes them “among the deepest diamonds ever found.”

This research, above all else, adds another chapter to an already insane story about the journey they go on before we dig them up. It’s more surprising than you probably realize, because diamonds aren’t what you think they are: compressed coal. So where do they come from?

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