Omphacite is a typomorphic mineral of eclogites, which is inappropriate to mineral assemblages of peridotites. Nevertheless, findings of this mineral in inclusions in peridotitic diamonds can be considered as indirect evidence for the existence of this paradoxical mineral assemblage. In this paper we present experimental results on the interaction between carbonate-bearing amphibolite and olivine that model processes operated at the crust–mantle boundary in subduction zones. The experiments demonstrate growth of omphacite at the interface between acid melt and peridotite media at 2.9 GPa and 850–900 °C; the omphacite coexists either with garnet and orthopyroxene or with phlogopite. The synthetic omphacite is exclusively of reactive-magmatic origin and does not form in metasomatic way. Findings of omphacite inclusions in peridotitic diamonds and in some pyroxenites from kimberlites are discussed in scope of the obtained experimental data.