Slake durability is a key parameter for evaluating the engineering behavior of weak rocks, particularly argillaceous rocks. Although the Slake Durability Index (SDI) is widely used to assess resistance to repeated wetting–drying cycles, it is based solely on weight loss and does not account for changes in the geometry or surface texture of rock fragments. This study introduces new aggregate surface-texture indices to improve characterization of the slaking process and the durability classification of weak rocks. A total of 28 rock samples from different lithological units in Golestan Province, Iran, comprising 267 rock fragments, were tested in accordance with ASTM D4644. In addition to SDI, changes in particle shape, geometric volume, and image-based elongation were quantified before and after testing. Four new indices were developed: Aggregate Form Change (AFC), Aggregate Volume Change (AVC), Aggregate Slaking Ratio (ASR), and Aggregate Distortion Change (ADC). Statistical analyses showed a very strong positive correlation between AFC and AVC and a significant negative correlation between AFC and the second-cycle Slake Durability Index (Id₂). According to the Gamble classification system, over 77% of the samples were classified as having very high durability (Id₂ > 95%), highlighting the limited discriminatory power of SDI for relatively durable weak rocks. In contrast, the proposed indices exhibited a broader range of variation and effectively distinguished differences in fragment degradation, even among samples with similar SDI values. These findings demonstrate that integrating AFC, AVC, ADC, and ASR with the conventional SDI improves the accuracy of durability assessment and engineering classification of weak rocks and provides a basis for developing a multiparametric classification system incorporating the geometric and surface-texture characteristics of rock fragments.