We relate generalized Lebesgue decompositions of measures along curve fragments ("Alberti representations") and Weaver derivations. This correspondence leads to a geometric characterization of the local norm on the Weaver cotangent bundle of a metric measure space $(X,\mu)$: the local norm of a form $df$ "sees" how fast $f$ grows on curve fragments "seen" by $\mu$. This implies a new characterization of differentiability spaces in terms of the $\mu$-a.e. equality of the local norm of $df$ and the local Lipschitz constant of $f$. As a consequence, the "Lip-lip" inequality of Keith must be an equality. We also provide dimensional bounds for the module of derivations in terms of the Assouad dimension of $X$.