Understanding coal's mechanical failure properties in the nano-to-micro scale is required to design safe and costefficient mining, hydraulic stimulation, and CO2 sequestration in coal seams. We propose a model to estimate the fracture toughness of coal based on nanoindentation. We use the brittle failure theory and fracturing-induced pop-in events in the loading curve to estimate the fracture energy and the fracture surface area. We conduct indentation studies on the Gondwana coal samples from India to validate our model. We use the model results to characterize the multiphase and multiscale nature of heterogeneity in the mechanical properties of Gondwana coal.