Diopside has been developed for use in dental root implants and for the filling of bone defects. In previous studies, diopside developed hydroxyapatite (HA) on its surface and achieved a direct bond with bone. The purpose of this study was to investigate the mechanism of crystal formation on the diopside surface. We ultrastructurally evaluated the interface between new diopside-induced crystals and diopside. Specimens were prepared in three experiments: (1) Granular diopside was immersed in simulated body fluid (SBF); (2) granular diopside was implanted into a cavity in rabbit bone; and (3) a diopside dental root implant was implanted into a Japanese monkey. The specimens were examined by contact microradiography, high-resolution transmission electron microscopy, and analytical electron microscopy. In the experiment with SBF, many platelet-like crystals formed in the diopside surface layer. The lattice of diopside and that of the new crystals were very close, but no clear continuation of the lattice was observed. In the experiments which used a rabbit and a monkey, contact microradiography showed close contact between bone and diopside. High-resolution transmission electron microscopy revealed crystal growth from the diopside surface layer, and continuity between the diopside lattice and that of the new crystals. The morphological characteristics of the new crystals and the results of these analyses suggest that these new crystals are HA. With regard to the mechanism by which crystals are formed on the diopside surface layer, it is possible that epitaxial crystal growth could originate as a nucleus on the surface. In this case, epitaxial crystal growth of primarily octacalcium phosphate (OCP) may have occurred, and this may have changed to HA by a phase transition. However, epitaxial growth of OCP on the diopside surface is still highly speculative, since there is no direct supporting evidence.