The size reduction process of rocks in cone crushers is one of the most important issues, particularly for the secondary and tertiary stages of crushing operations. In this study, 17 different rock types were considered for the evaluation of their size reduction variations that occurred in a laboratory-scale cone crusher. Based on several mineralogical, physico-mechanical, and aggregate properties determined for each rock type, the crushability tests were performed. Before and after the crushability tests, particle size distribution (PSD) of the uncrushed (feed) and crushed (product) materials were determined by sieve analyses. On the basis of these PSDs, the degree of rock crushability (DRC) was attempted to quantify by simple approaches (i.e., size reduction ratio, SRR, and the theoretical square mesh aperture size that corresponds to the 10% of the cumulative undersize in the product, P-10 (mm)). The crushability test results demonstrated that the DRC in cone crusher could be quantified by focusing on the variations in the SRR and P-10. The SRR and P-10 are associated with three important rock properties, Shore hardness (SH), Los Angeles abrasion loss (LAA, %), and Brazilian tensile strength (BTS, MPa). The textural and mineralogical features of rocks also have substantial impacts on the DRC for several rock types. It was concluded that the combination of the SRR and P-10 could be considered together for the evaluation of DRC in cone crushers. Moreover, further research potentials on the DRC were also discussed in this study.