Several cracked chevron-notched Brazilian disk (CCNBD) and hollow center cracked disk (HCCD) samples of crystalline building stones naming Maroon, Baghat, and Neiriz in Iran, bearing the same calcite mineralogy and different grain size distributions were numerically modeled using three-dimensional particle flow code, with the respective microscopic mechanical properties measured through calibration technique. The simulation of CCNBD and HCCD samples highlights higher toughness compared to laboratory measurements. The values regarding HCCD samples are in a better agreement with experimental results than those regarding the CCNBD samples. The principal objective was to evaluate the reliability of numerical simulations in estimating fracture toughness and to recognize the mechanism and the mode of failure, of which laboratory tests are incapable. At microscales, failures occurring due to tensile or shear loadings may not be subsumed within any category as such at macroscales with failure modes I and II. Experimental and numerical results show that the K-IC values determined from both CCNBD and HCCD samples have a negative correlation with the grain size distributions of the marbles. However, no specific trend as such was identified for K-IIC.