The increasing demand for underground structures necessitates a comprehensive grasp of rockmass behaviour to ensure safety. Characterising deformation processes at the rockmass scale poses significant challenges. To address this, rockmass analogues are valuable for gaining insights into rockmass behaviour. Accordingly, this study focuses on development of a laboratory-scale analogue of a non-persistently jointed rockmass by inducing diffused (stochastically distributed) damage via thermal treatment. For this, intact specimens (~50–150 mm) of Agaria marble were subjected to controlled thermal loads, reaching predetermined target temperatures of 560 °C at a heating rate of 3.5 °C/min, which was maintained for 4 h. The modifications in the volume of the rock specimens were quantified via evaluation of the changes in mass, uniaxial compressive strength (UCS) and microstructure. Novelly, the thermal load induced full-field strains were also studied via an image correlation system to allow advanced insights into spatial characteristics of diffused damage post thermal loading, to evaluate the extent of damage in the rock volume. The study concluded that thermal treatment induced observable damage in rock specimens, as evidenced by diminished mass, UCS, and microstructure. Furthermore, the full-field strains quantitatively showed the spatial dispersion in damage, which is consistent with the structure of a non-persistently jointed rockmass.