Scientific American has a story about the formation of diamonds. Inclusions are imperfections in gem stones. They are tiny bits of the surrounding rock when the gem or crystal forms. Inclusions in two diamond samples from South Africa are shedding light on how diamonds form deep in Earth’s mantle.

The two new diamond samples each contain inclusions of carbonate minerals that are rich in oxygen atoms (a state known as oxidized) and oxygen-poor nickel alloys (a state known as reduced, in the parlance of chemistry). Much like how an acid and a base immediately react to form water and a salt, oxidized carbonate minerals and reduced metals don’t coexist for long. Typically, diamond inclusions show just one or the other, so the presence of both perplexed Yaakov Weiss, a senior lecturer in Earth sciences at the Hebrew University of Jerusalem, and his colleagues—so much so that they initially put the samples aside for a year in confusion, he says.

But when they reanalyzed the diamonds, the researchers realized that the inclusions capture a snapshot of the reaction that made the sparkling stones and confirm for the first time that diamonds can form when carbonate minerals and reduced metals in the mantle react. The new samples are the first time scientists have ever seen the midpoint of that reaction captured in a natural diamond.

“It’s basically two sides of the [oxidation] spectrum,” says Weiss, the senior author of the new study describing the find, which was published on Monday in Nature Geoscience.