Recent analysis of two small gemstones suggests that the famous Hope and Cullinan diamonds were formed more than 373 miles deep.

Most of the Earth’s natural diamonds are formed at the bottom of tectonic plates (between 93 to 124 miles deep), where the crust meets the more fluid outer mantle of the planet. Some of these precious stones, developed under immense pressure, then rise to the surface, enough to be sometimes discovered.

The “Hope” diamond and the “Cullinan” diamond, two of the most famous diamonds, on the other hand, seem to have had a different history. According to new research, these two stones have indeed developed much more deeply.

Over 410 miles deep: All diamonds, formed of carbon crystals, have various chemical impurities. The type of stone is then determined by the impurities embedded during its development. Thus, it is assumed that two diamonds of the same type were probably formed under similar conditions. In the context of new work, presented at the Goldschmidt 2020 geochemistry conference, the researchers explain having analyzed two less famous diamonds.

The first is a 20-carat “type IIb blue diamond” discovered in South Africa. The “Hope” (45.52 carats) is a larger version of the same type of diamond. The other object studied is a 124-carat stone the size of a walnut called the “CLIPPIR” diamond (for Cullinan-like, Large, Inclusion-Poor, Pure, Irregular, and Resorbed). Discovered in Lesotho, a landlocked kingdom in the territory of South Africa, this diamond resembles, as its name suggests, Cullinan.

By analyzing these two stones, the researchers explain that they detected the presence of bridgmanite inside. It is a common mineral found in the lower mantle, a fluid area extending from 410 miles deep to the liquid outer core of the planet. The discovery of such minerals trapped in these diamonds means that they have crystallized in the lower mantle.

Based on the principle that these two objects studied are of the same type as the Hope and Cullinan diamonds, this new study then suggests that the two large stones are also from the same environment.

The Cullinan, discovered on January 26, 1905 in the Premier mine, located near Pretoria (South Africa) is none other than the largest rough diamond ever discovered (3,106 carats). Colorless, it was quickly split into nine huge main stones (Cullinan I to IX).

The Cullinan I (530.2 carats) today adorns the British imperial scepter, displayed with the jewels of the British Crown at the Tower of London. The Cullinan II (317.4 carats), for its part, is set on the front face of the Imperial state crown, one of the other jewels of the British Crown.

The origins of the Hope diamond, on the other hand, are vague. According to the Smithsonian Institution (the current owner of the diamond), it was discovered by miners in India, before being sold to the French merchant Jean-Baptiste Tavernier in 1668.

The Hope would then be passed into the hands of the French royal family, wealthy British collectors, and then American businessmen, before finally being exhibited at the National Museum of Natural History in the United States. For your information, it is now the second most visited art object in the world after the Mona Lisa.

Hugues Louissaint

Hugues Louissaint is an entrepreneur and writer, living in the US for over a decade. He has launched successful products such the Marabou Coffee brand, which has been highly successful in Florida. He has also been a writer for more than 5 years focusing on science, technology, and health. He writes part-time for the Scientific Origin and provides valuable input on a wide range of subjects.