It is widely assumed that mineral inclusions and their host diamonds are 'syngenetic' in origin, which means that they formed simultaneously and from the same chemical processes. Mineral inclusions that, instead, were formed earlier with respect to diamonds are termed protogenetic. However, minerals can have the same age as the diamonds in that they become enclosed in and isolated from any further isotopic exchange. But this is termed 'synchronous' not 'syngenetic'. Here we demonstrate conclusively the protogenesis of inclusions in diamonds, based upon data from an exceptional fragment of a diamond-bearing peridotite, its clinopyroxene and a gem-quality diamond. Clinopyroxenes in the xenolith had the same chemistry and crystallographic orientation as those for inclusions in the diamond. With our results with garnets, olivines and sulfides, we can state that a major portion of the mineral inclusions in non-coated, monocrystalline-lithospheric diamonds are protogenetic. Our discovery here presented has implications for all genetic aspects of diamond growth, including their ages.