Source data for Figures 1–5 of the manuscript "Shape evolution of pumice during granular flow" (submitted to Communications Earth & Environment). The dataset includes results from controlled tumbling experiments (Fig. 1), morphological parameters of starting material and tumbled clasts — solidity, convexity, form factor, axial ratio, and volume — for Set A (Fig. 2) and Set B (Fig. 3), bivariate trends (form factor vs. axial ratio) for both sets (Fig. 4), and mean values of shape parameters (form factor, solidity, convexity) used in the application of relaxation theory (Fig. 5). These data enable full reproduction of the analyses and figures in the manuscript.