Diamonds are highly prized for their qualities but also for their rarity. One way to look for them is to search for associated minerals that occur more commonly, such as the chromium-rich pyrope garnet.

This vibrant purple garnet is easily found by diamond exploration companies, in sediment downstream from potentially diamond-bearing volcanic pipes, and within the pipes themselves. The presence of purple garnet is an indicator diamonds may also be present.

Moreover, this garnet isn't just found near diamonds, but is also consistently found inside them. So by enhancing our understanding of pyrope garnet, and how it forms, we can also enhance our understanding of diamond formation.

It was previously thought this type of garnet could not form very deep in the Earth. The theory went that it originated from a different chromium-rich mineral, called spinel, which formed at a shallow depth in the mantle and was then pushed down where temperatures and pressures were higher—leading to the garnet's formation.

Our latest research, published on March 15 in Nature, uses a new model to revisit an old theory that suggests these pyrope garnets are actually formed much deeper in the mantle, about 100km-250km below the present surface. It also suggests diamonds may be rarer than we think.