The modification of the (111) face of synthetic diamond under high-fluence (≥1018 ion/cm2) 30-keV Ar+ irradiation is studied experimentally. It is found that ion irradiation at room temperature results in the formation of a low-conductivity surface layer. Heat treatment when the target temperature is increased to 400°C results in a more than ten-fold exponential drop in the layer resistance, as compared to its value at room temperature. If the temperature of the irradiated diamond is increased from 30 to 400°C the layer resistance of the ion-induced conductive layer drops by more than two orders of magnitude to the level corresponding to the conductivity of graphite-like materials. The Raman spectra of the ion-induced conductive surface layer reflect the processes of structural disorder—sp2-carbon ordering and strong changes in the optical transmittance of diamond after ion irradiation.