We can only guess at the minerals deep in the mantle

What actually happened

Davemaoite is one of the three main minerals that make up the lower mantle, estimated at 5 to 7 percent of that vast region, and it is important out of proportion to its abundance because its crystal structure hosts heat-producing radioactive elements like uranium, thorium, and potassium. In other words, this mineral is part of the planet's internal furnace, and until 2021 no one had ever held any.

The problem is that calcium silicate perovskite is unstable at the surface: bring it up from 660-plus kilometres and the drop in pressure destroys its structure before it reaches you. The only way to see it intact is to have it sealed inside something rigid enough to preserve the deep pressure, and a diamond does exactly that. This particular diamond, from the Orapa mine, had been sitting in a collection since 1987 before the inclusion was identified.

The correction is to a statement of hard limits: that direct samples of the lower mantle are impossible and its mineralogy is inference only. As of 2021 that is no longer strictly true. It took a natural pressure vessel and a lot of luck, but a predicted deep-Earth mineral was finally caught in the flesh and given a name, and the International Mineralogical Association added it to the official list.