Diamond
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Blue diamonds are the rarest type of naturally coloured diamonds on Earth and they can only be formed hundreds, if not thousands of kilometres below the surface, deep in the crust and they require complex geological processes to happen in sequence for the colour to come through.

A new study carried out on diamonds has found that gems like the Hope Diamond, one of the most famous and notoriously believed to be cursed pieces of jewellery had to go through almost hellish heat and pressure, deep below the surface, for hundreds of years before they are spewed to the surface.

Blue diamonds are the rarest diamonds on Earth. According to a recent survey, of the 13.8 million diamonds found, only 0.02 percent were blue. Scientists have found that the blue version of these gemstones forms four times as deep inside the Earth as the clear variants. The new study indicates that the required depth should be at least 643 km below the surface.

Blue diamonds have long been a puzzle to geologists, says Evan M. Smith, a lead author of the study. "We knew essentially absolutely nothing about where they grow," he said. In their most basic form, they're crystals of carbon atoms. So any colouration is an impurity in the crystalline structure of the diamond. Blue diamonds say geologists can be traced to boron contamination that gets trapped when diamonds grow on rocks—the way moss grows on wood.

Boron atoms can replace carbon atoms within crystal structures, though this substitution is not perfect, and in the exchange process, boron loses electrons, absorbing red light, which in turn gives the gem a blue-ish hue.

However, boron is found on the surface of the Earth, not necessarily at the depths where diamonds form, note the researchers. To try and answer the question of where the boron came from, Smith and his team reviewed 46 blue diamonds submitted to the Gemological Institute of America. Researchers did not include the Hope Diamond itself in this study, said a report published in Washington Post.

When a diamond grows, it can, at times envelop material surrounding it and trap it. This can be likened to bugs being caught inside amber. When carbon crystals are able to trap certain minerals within them, they are known as inclusions. Blue diamonds have an unusual birthplace and history to tell the world about the planet, especially the surface oceanic crust and the underlying ocean mantle.

The movement of tectonic plates forces slabs of ocean crust to descend into the depths of the Earth for hundreds of kilometres. This explains boron present in unusual depths because seawater is known to contain boron.

Though this theory cannot conclusively prove the origin of blue diamonds, scientists cannot prove it unless the diamond is cut open and studied, but then the value of it is lost forever. The study was published in the journal Nature.