The world’s largest pink diamond mine

The Argyle mine contains the largest amount of pink diamonds ever discovered on Earth. Unlike blue and yellow diamonds, which result from impurities such as nitrogen and boron, pink diamonds’ color comes from geological processes that deform the crystal structure. Pink diamonds are extremely rare and can be worth more than $2 million per carat (1 carat equals 0.2 g), according to the International Gemological Association.

The Argyle mine closed in 2020 due to a decline in diamond supplies and unfavorable economic conditions, including increased operating costs. The mine is located on the shore of Lake Argyle in a remote area of ​​Western Australia state, 550 km southeast of Darwin. Open mining there lasted 37 years and produced more than 865 million carats (172 tons) of rough diamonds, including white, blue, purple, pink and red diamonds, according to Rio Tinto, which owns and mine operations.

The Argyle Rock Formation is an unusual diamond-forming location because it is located on the edge of a continent rather than in the center, where gems typically occur. Also, diamonds are always found in kimberlite rock formations, but the Argyle Formation contains a type of volcanic rock called olivine lamproite.

Researchers dated the rocks at Argyle shortly after they discovered the site in 1979. Initial results put the rocks at around 1.1 to 1.2 billion years old, but last year, new research revealed that the rocks are 1.3 billion years old. This means the Argyle Formation was formed just as the Nuna supercontinent began to break up, revealing clues about how diamonds form and why there are so many pink diamonds.

Pink diamonds are created under special temperature and pressure conditions when tectonic plates collide. The extreme force of these collisions can bend the crystal lattice of pre-existing diamonds in ways that give them different pink tones. If the force is too great, they can turn brown, according to Hugo Olierook, a PhD student at Curtin University, Australia, author of the 2023 study.

The Nuna supercontinent formed when two parts of the Earth’s crust crashed into each other 1.8 billion years ago. The area where they collided coincides with today’s Argyle Formation, proving that the collision led to the birth of pink diamonds here. However, at that time, diamonds were buried deep in the Earth’s crust.

About 500 million years later, as Nuna began to break apart as tectonic plates moved apart, the diamond-bearing rock was pushed closer to the surface. That rock also contained an abundance of brown diamonds, which Rio Tinto mined and sold in huge quantities.