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Seed protection is a key element in plant survival and plant dispersion. In hazelnuts, shell lignification begins from the tip to the basal scar. Therefore, hardening time is a race between seed survival against environmental stress. While new pests and stochastic weather events are increasing, fundamental shell lignification features are missing in the common Segorbe cultivar. In this purpose, progress of lignification was studied using hardness and morphological changes in fresh and dried Segorbe hazelnuts collected weekly in 2020 and 2021, respectively. The growing degree days (GDD) were calculated for key lignification process stages. In dried hazelnuts, the lignification process causes morphological deformation lead to belted hazelnut symptoms. Lignification increased progressively from week 22 (1353.6 GDD) to week 26 (1353.6 GDD). Shell length was 48–84% of their final size. The shell was fully lignified at week 27 with a kernel size of 4.99±0.32 mm. In fresh nuts, shell size increased from 83 to 92%, and hardening increased by a factor of 2.85, reaching 21 kgf, between weeks 25 and 26 (2272.8 GDD), with a kernel size of 4.75±2.05 mm. Lignification occurs in four weeks. When kernel size is ~5 mm, and shell growth ends, a hard wall protects the kernel from stressors. Belted hazelnut symptoms, reflected in the ratio of lignified and shriveled shell parts, and shell hardness, are discussed in the context of the susceptibility of Segorbe cultivars to environmental stressors.