It is shown that large and well faceted hexagonal crystallites can grow on Si and Ti substrates under higher nitrogen gas flow in the gaseous mixture of CH₄ and H₂ in the normal process of diamond deposition using microwave plasma chemical vapour deposition (MP-CVD). Grazing incidence x-ray diffraction (GIXRD), energy dispersive x-ray spectrometry (EDX), wavelength dispersive x-ray spectroscopy (WDX) and Raman analysis revealed that these crystals are silicon carbonitride rather than carbonitride as we expected and usually regarded. This indicates that the hypothetical covalent carbonitride is not as easily formed as the crystallite with Si substitution. Comparing films deposited on Ti alloy substrates, with and without Si chips present during the deposition, we found that the Si chips activated by the plasma sputtering provides Si sources for crystallite SiCN formation on Ti substrate. The hexagonal SiCN structure and its lower hardness are attributed to the C_3v-symmetric quasi-tetrahedral nitride with a nonbonding lone pair. The involvement of the less electronegative Si specimen may make the SiCN form easier than the covalent carbonitride.