Primarily, I use stable isotope geochemistry (C-N mostly, but recently using O and Noble gases) to address the origin of diamond-forming carbon in the mantle; see publications page for my contributions).
The petrological and geochemical interrogation of mantle diamonds and their inclusions provide the best empirical method to study the deep carbon cycle through geological time. For the geological purposes of tracing the history of C-bearing fluids, there are three main diamond types; monocrystalline, coated, and polycrystalline (shown below).
Monocrystalline diamonds are the most abundant diamond type, and they are the group from which gem-quality samples are obtained. Monocrystalline diamonds are most commonly observed as octahedral single crystals, and they are an important source of mantle silicate, oxide, sulfide, and metallic inclusions. Furthermore, they form the basis for the geochronological record of diamond-formation, which appears episodic from the Archean to the Proterozoic. Coated diamonds are also single crystals but instead comprise a separate category due to their composite and fluid-rich nature. Coated diamonds consist of an older monocrystalline core overgrown by thick, cloudy, younger diamond coat, rich with microscopic fluid-solid inclusions. These coats are currently thought to have formed either from proto-kimberlitic fluids or via hydrous metasomatism of the sub-cratonic mantle by hyper-saline fluids derived from a subducting slab. Finally, there are the polycrystalline diamonds, which are found as either framboids (i.e. boart) or diamondite, the distinction is made where the former is pure diamond and the latter is predominantly comprised of diamond and silicate minerals, (the focus of my own research).