Summary: When diamond is synthesized at conditions of comparatively high temperature and pressure, the nucleation rate is high, as is the growth rate of the nuclei. Consequently the product is usually an aggregate of crystals with dendritic or skeletal structure. In this study the presence of gold or silver as an additive mixed with a catalyst was found to have the effect of suppressing nucleation. When a homogeneous mixture of graphite, catalyst, and additive was treated at conditions where skeletons and dendrites were produced in the absence of additive, euhedral crystals of octahedra were formed. When a special cell assemblage for high pressure experiments, in which the graphite was placed inside a cylinder of catalyst coated with additive, was used, prismatic and tabular crystals were synthesized.