The investigation of minerals found as inclusions in natural diamonds, showing a range of pressures of formation from 5 to 30 GPa (equivalent to depths of ca 150 to 800 kms). In addition to determining the chemical composition and structure of the minerals, the circumstances of diamond formation are explored, and their possible relationships to the distribution of water and melts are investigated. A close association of the deeper diamonds with slabs of subducted ocean crust is being documented.