<p>The brittleness of rocks significantly impacts the rock-breaking machinery and tunnel excavatability. By considering the energy conversion mechanisms, a new rock brittleness index (<i>BI</i>_<i>N</i>) to evaluate tunnel boring machine (TBM) tunneling efficiency is proposed in this study. With the Longyan Wan’anxi diversion tunnel as an example, the proposed index is used to study the differences in brittleness of granite and sandstone and the effects of moisture conditions, and the relationships between <i>BI</i>_<i>N</i> and TBM are analyzed as well. The results indicate that the proposed index can be used to characterize rock brittleness effectively. Greater rock strength and lower mechanical energy result in increased surface area of rock chips per unit volume after breaking (i.e., greater brittleness), and brittleness decreases with the increase of moisture content. Granite generally exhibits greater brittleness than sandstone, while sandstone shows more significant negative correlation between brittleness and moisture content. Specifically, the <i>BI</i>_<i>N</i> of granite and sandstone had decreases of 32.86% and 42.11% respectively after saturation. Besides, comparative analysis of other brittleness indices reveals a good correlation between the proposed brittleness index and TBM specific energy (SE). Furthermore, the relationship between <i>BI</i>_<i>N</i> and Cerchar Abrasivity Index (<i>CAI</i>) for the two types of rocks under dry and saturated conditions was explored respectively. For sandstone, a linear relationship between BIN and CAI is observed regardless of being in a dry or saturated state.</p>