I found this article pretty interesting: The GOPE 25 Kimberlite Discovery, Botswana, Predicated on Four Mg-Ilmenite Grains from Reconnaissance Soil Samples: A Case History
Basically, some kimberlites were identified using indicator minerals that came from the soil above the pipes. One of the important indicator minerals was ilmenite (which I had in my own pet rock from Green Mountain). Even more interesting, the scientists used the chemical signatures of these ilmenites to infer if the kimberlites in question were likely to contain diamonds. That’s some pretty cool stuff.