Hydraulic fracturing in unconventional shale formations relies on factors like shale brittleness for creating an effective fracture network directing gas to the wellbore. This study assesses the brittleness of five shale varieties from Rajmahal Basin, India, through strength properties, elastic properties, stress–strain response, mineralogical composition, and total organic carbon content (TOC). Shale varieties used in this study were: (i) Sandy Shale (SST-SH); (ii) Shale with sand laminations (SST-Lam-SH); (iii) Coaly Shale (C-SH); (iv) Intercalated Shale (ICAL-SH); (v) Carbonaceous Shale (Cb-SH). Brittleness index measured based on mineralogy (BIm) revealed that ICAL-SH, SST-SH, and SST-Lam-SH samples, with dominant clay mineral content, are less brittle than C-SH and Cb-SH samples with higher quartz content but little to no clay content. Additionally, high-TOC shales (C-SH and Cb-SH) were observed to be more brittle compared to low-TOC shales (ICAL-SH, SST-SH, and SST-Lam-SH). Similar variation of brittleness with shale varieties was observed for BI measured based on strength properties (BIUCS/BTS). However, a complete opposite trend was shown by BI of the shale rocks measured based on elastic properties (BIE/V). Low-strength samples were found to be more brittle than samples exhibiting higher strength. Stress–strain curves of the brittle shales (C-SH and Cb-SH) showed sustained plastic deformation before failure compared to less-brittle shales (ICAL-SH, SST-SH, and SST-Lam-SH). Failure pattern of the shale samples under uniaxial compression was consistent with the measured brittleness indices: C-SH and Cb-SH samples exhibited tensile failure, whereas ICAL-SH, SST-SH, and SST-Lam-SH samples exhibited shear failure.