The internal structure and growth history of diamonds have been investigated by the study of carbon isotopes and the abundance, isotope composition and aggregation state of nitrogen. Polished plates of zoned diamonds (“coated stones”) were laser sectioned for infrared mapping and isotope profiling. Nine small cubic and three small octahedral diamonds from Zaïre and two coated stones and five small cubic diamonds specifically from the Mbuji Mayi area of that country were also investigated. The isotope composition and infrared characteristics of the diamonds of cubic habit, including coats, were found to be indistinguishable with δ¹³C in the range −5.9 to −7.5‰ and all the nitrogen being present as the paired structure (IaA) with a δ¹⁵N of between −3.1 and −8.0‰. The diamonds of octahedral habit, including the cores of the coated stones, were more variable: δ¹³C varied from −4.9 to −9.8‰. Infrared spectra of the octahedral diamond exhibited either strong IaA characteristics or a combination of IaA and IaB. Platelets were present in most cases. δ¹⁵N values from octahedral diamonds varied from −5.8 to +13.4‰. The data suggest that the octahedral diamonds represent a variety of growth events separated in time and/or space and that the diamonds resided in the lithosphere for a period sufficient for thermal maturation. In the case of the coated stones the octahedral diamonds were later involved in an event which added a component from a single relatively homogeneous reservoir; cubic habit diamonds were formed at the same time. This latter event is believed to be related to the kimberlite eruption. Comparison of the data with carbon isotope analyses of cubic diamond from other sources suggests that this reservoir could be widespread.