March 31, 2020

Blue Diamonds: The Heart of the Ocean

Coloured diamonds have been at the top of wish lists for centuries, their natural beauty and rarity has caused many hearts to skip a beat. Whenever blue diamonds come up at auction they are always a hot topic, often selling for millions of dollars per carat, they continue to rise in popularity and price.

There is much mystery surrounding coloured diamonds, why they are rare, why are they only found in certain places and what makes them that colour? The most important blue diamonds have come from just three sources, India, the Cullinan Mine in South Africa and the Argyle Mine in Australia in areas which the oldest tectonic plates are (cratons).

Research published in Nature in 2018 (a Science journal) has given us a window into the unanswered questions surrounding these magnificent gems.

It’s known that it is the presence of boron in the crystal lattice structure of a diamond which makes it blue, but where does this boron come from? Boron is a rare Earth element, and in nature is never found in its elemental state, due to the high oxygen atmosphere of the Earth, it is usually found in an oxidated state as Borate, however Boron has a high concentration in Continental and Oceanic crust.

Analysis of 46 blue diamonds showed that the inclusions (trapped minerals) in the stones were of rocks only found in extreme temperature and pressure circumstances, specifically those of the Earth’s lower mantle.

The research showed that blue diamonds are formed between 410-660km below the surface of the earth, in the transition zone between the upper and lower mantle. The range of depths found showed that blue diamonds can form anywhere between >660km-150km.

These findings have led scientists to believe that blue diamonds that are found in Kimberlites (diamond bearing volcanic rocks), which were formed as a result of a subduction zone. It’s been hypothesised that boron-containing sea waters at subduction zones is carried down into the mantle, (there is a relatively high concentration in the crust and low concentration in the mantle), and incites the formation of diamonds.

Previous studies have shown that the largest and most valuable colourless diamonds have come from “super-deep” depths, coupled with results of this paper, we now know that the Earth’s mantle is a production line for the worlds’ finest gems.