## 7 Challenges and Future Research Directions  
Brittleness is a crucial rock property that significantly influences both the failure behavior of intact rock and the overall response of rock masses in tunneling and mining operations [85]. However, various authors define, interpret, and apply the concept differently.  
Different brittleness indices have different limitations in describing how rocks fracture, and they are presented in this section. For example, B1 and B2 are more closely associated with rock strength than brittleness, making B1 more appropriate for characterizing strength rather than brittleness. B3 to B5 share similar limitations to B1 and B2 but show relationships that are useful in drillability. B21 is developed for shale, but it is also not perfect as it increases with peak strain, which does not always indicate higher brittleness. B72, used in crack formation studies, has challenges in measuring deformation due to its dependence on load and the complexity of separating rock volume deformation from crack deformation.  
Brittleness indices mainly focus on mechanical properties like strength and hardness, not focusing on the key petrographic factors such as mineral composition, grain size, and microstructures. This can lead to inaccurate assessments as these characteristics significantly influence rock brittleness and fracture behavior [66; 122]. When overlooking these petrographic factors, standard brittleness indices may misrepresent norite's true behavior, leading to errors in rock engineering, drilling, and reservoir characterization [67].  
Incorporating both petrographic and mechanical data of a rock provides a more comprehensive understanding, enhances predictive accuracy, and helps resolve the brittleness and determine the complex geological conditions of a rock. [PERSON] et al. [7] (2022) further outlined that combining petrographic and mechanical data offers a more accurate understanding of rock behavior. This also enhances predictions of brittleness, failure patterns, and rock performance, improving decision-making and outcomes in engineering applications like drilling, mining, and reservoir management.  
Future efforts should focus on developing models and equations that account for the combined effects of textural features on mechanical properties through both experiments and numerical simulations, validating these models for rocks from various origins, and applying these insights to predict mechanical properties more accurately.  
Field studies are crucial for validating laboratory findings as they provide real-world data that reflect the actual conditions of the site. It is critical to understand the phenomenon of rock burst, with an emphasis on identifying its occurrence patterns. Thus, acquiring this knowledge is vital for preventing or managing procedures in mining, tunneling, and construction projects, ultimately saving both costs and lives [123]. This information can also be used to optimize designs and develop more effective techniques for excavation, support systems, and stability assessments. Ultimately, field studies bridge the gap between theoretical research and practical applications, ensuring better decision-making and more successful project outcomes.