Colourless olivine inclusion in a natural rough octahedral diamond. Image by Stefano Castelli (University of Padova).
Natural diamonds often contain mineral inclusions. These are pristine pieces of the deep Earth that were trapped inside the diamond as it grew.
Did the diamond trap a pre-existing mineral from the surrounding rock (protogenetic)? Or did the mineral co-crystallise from the same fluid as the diamond (syngenetic)? Why is this distinction important, and how can we tell the difference?
Fabrizio Nestola and his co-authors have published a few research articles that investigate whether inclusions are protogenetic or syngenetic, and you can read those here and here and here.