Pyrometric interferometry has been performed in the formation of diamond crystals and films synthesised on pretreated single silicon substrates by a microwave plasma assisted chemical vapour deposition (MPCVD) technique. The growth mode of this material induces particular variations of the emissivity ratio epsilon/epsilon(0) that we an able to simulate theoretically, taking into account the nucleation mode (Volmer-Weber), the nucleation density N and the growth rate nu of the diamond crystallites. The comparison between the experimental and the theoretical curves of the epsilon/epsilon(0) variations allows us to understand better the first stage of the CVD diamond formation in terms of kinetic and nucleation density. Finally, the real-time monitoring of these parameters is a very attractive objective for the precise control of the deposition process, for which the first stages are crucial for the subsequent diamond film quality.