In the early 1980s, the Orapa mine in Botswana, the largest open-cast diamond mine in the world, produced a 428-carat diamond.

Despite its size, the stone was unsuccessful in the rough diamond market.

The gemstone has a greenish tinge and there are several dark spots inside, which result from inclusions of other minerals.

That reduced its sales value in the jewelery market considerably.

The stone passed through several hands before it was sold in 1987 by a diamond dealer to a mineralogist at the California Institute of Technology in Pasadena, a suburb of Los Angeles. There George Rossman wanted to find out with his students why some diamonds appear green and not "crystal clear". The experiments showed that the gemstone contains inclusions of carbon dioxide (CO₂), but they did not provide a conclusive answer to the question of color. The diamond ended up in a drawer at the college in Pasadena.

The gemstone slumbered there until almost ten years ago, when the German mineralogist Oliver Tschauner, who is now teaching at the University of Nevada in Las Vegas, wanted to investigate the issue of CO₂ inclusions.

Together with Rossman, he exposed the diamond to intense X-rays in order to learn more about the nature of the mineral dark spots from its diffraction pattern.

Although the researchers did not learn anything about carbon dioxide, there was a small sensation: The gemstone contained “Ice VII”, an extremely rare high-pressure phase of conventional water ice.

Calcium silicate in a special high pressure form

Even after this discovery, which surprised the experts, Tschauner did not let the inclusions in the gemstone rest. After all, they are like a window that allows a glimpse into the chemical and physical state of the earth's mantle, the birthplace of most diamonds. These carbon compounds are created under enormous mechanical pressure at depths of several hundred kilometers and then reach the surface of the earth via volcanic vents, the so-called kimberlites. Southern Africa in particular is rich in these long-extinct remains of volcanoes.

At the synchrotron of the Argonne Laboratory near Chicago, Tschauner exposed a small, polished disc of the diamond to strong X-rays. And there was another surprise: some of the inclusions were rich in the element calcium. Tschauner has now been able to confirm with various other methods: The diamond contains a mineral that researchers had previously believed could not exist at all on the earth's surface. It is a calcium silicate (CaSiO₃) of the mineral form of perovskite. This mineral is only created under extremely high pressure, such as that found in the lower mantle at a depth of almost a thousand kilometers.

Just like the previously discovered Ice VII, this perovskite was captured by the carbon in the formation of the diamond in the deep earth's mantle. Contrary to all expectations, however, it then survived the several hundred kilometers long journey through the mantle and the rocky crust of our planet unscathed. Instead of disintegrating on the upward journey as the pressure decreased, the mineral remained in the diamond as a tiny dark spot.

The find, about which Tschauner and his colleagues are now

reporting

in the journal

Science

, confirms a previously only theoretical assumption made by geoscientists about the internal structure of the earth: in the lower mantle, below 660 kilometers deep, most minerals occur only as perovskites. This applies to the mineral made of magnesium-iron-silicate, also proven by Tschauner, the mineral bridgmanite and the calcium silicate that has now been found. Together, these two perovskites make up more than 40 percent of the volume of the earth's mantle.

The new perovskite mineral has now also had a name.

It was named Davemaoit after one of the pioneers of high pressure mineralogy, Ho-Kwang (Dave) Mao.

The diamond is now on display at the Museum of Natural History in Los Angeles, where it is accessible to all researchers for further investigation.

Who knows what other surprises he'll have in store.