The eruption of deep-seated xenoliths in basaltic, alnoitic, kimberlitic, etc volcanoes provides the geologist with an important direct means of examining the fragments of the earth's mantle and lower crust.

Diamond is the record-setter in many mineralogical properties such as hardness, diffusivity, thermal conductivity, purity, and covalency of bonding. Similarly, diamond, as the premier gemstone of the mantle holds primacy for geological features such as age and depth of origin. Diamond was among the first crystalline structures to be solved by X-ray diffraction and the first materials measured for their Raman spectrum. At more than 80 billion USD in yearly commercial value, diamond sets the record for the most traded, valuable mineral on the planet. Despite its chemical simplicity, diamond has been the object of more research effort, and had more scientific and popular press pages written about it, than any other mineral.

"Carbon in Earth is an outgrowth of the Deep Carbon Observatory (DCO), a 10-year international research effort dedicated to achieving transformational understanding of the chemical and biological roles of carbon in Earth (http://dco.ciw.edu). Hundreds of researchers from 6 continents, including all 51 coauthors of this volume, are now engaged in the DCO effort. This volume serves as a benchmark for our present understanding of Earth's carbon - both what we know and what we have yet to learn. Ultimately, the goal is to produce a second, companion volume to mark the progress of this decadal initiative.