Abstract <p>The effect of spontaneous nucleation of the diamond phase on cerium-doped yttrium aluminum garnet (YAG:Ce) particles is observed under the action of a microwave plasma in a methane‒hydrogen gas mixture at a substrate temperature of 820°C. These conditions facilitate the process of “chemical vapor deposition (CVD) of diamond” (CVD), but the synthesis of diamond films requires additional procedures to stimulate nucleation. It is shown that YAG:Ce particles can be used as diamond nucleation centers. After the 20-min CVD process, diamond particles of different sizes and a density of ≈10⁶ cm^‒2, which is 15‒20% of the deposition density of YAG:Ce particles, are detected by SEM. Luminescence bands from Ce³⁺ ions (550 nm) and Si-V centers in diamond (738 nm) are detected in the photoluminescence spectra of the synthesized diamond crystallites. The observed effect is interesting for studying the processes of diamond phase growth by the CVD method, especially in the synthesis of diamond composites for the tasks of visualizing high-power X-ray radiation.</p>