This study experimentally investigates and numerically models the charging behavior of a high-temperature rock thermal energy storage (RTES) system packed with crushed dolerite and operated up to approximately 600 °C. A one-dimensional, two-phase axial model is developed with temperature-dependent material properties, while radiative heat transfer is incorporated through an added radiative contribution to the solid effective thermal conductivity. The model is validated against measured temperature profiles, showing good agreement with the experiments. After validation, we quantify the sensitivity of thermal stratification to the effective conductivity by examining three representative values (0.2, 2, and 20 W/m·K). Results show that low to moderate radiative enhancement slightly broadens the thermal front but preserves a usable thermocline. In contrast, a high effective conductivity accelerates thermal diffusion during standby, flattens the axial temperature gradient, degrades the thermocline, and shortens the duration of useful high-temperature outlet conditions during discharge, even when the total stored energy remains comparable. These findings highlight that reliable estimation of effective thermal conductivity, including its radiative component, is crucial for accurate standby performance prediction and for robust RTES design. This becomes even more critical for long duration energy storage application. Finally, we assess design strategies to mitigate excessive thermal spreading and enable longer storage durations when the effective conductivity of the bed is large. The findings are expected to benefit the thermal management of high-temperature energy storage applications.