Scanning electron microscopy of calcium oxalate stones indicates a zonary structure of the Weddellite crystals of urine concrements which is due to concentration-dependent growth oscillations representing the precipitation of various calcium oxalate hydrates. In addition to the phenomenological demonstration of Weddellite crystal shapes, incubation experiments with different solutions give rise to the postulation of a dehydration process of primary Weddellite crystals via dissolution and recrystallization to Whewellite. The solvent attack starts at energetically preferred corners, edges and lattice disorders of the Weddellite crystals. The Weddellite-stabilizing role of urinary Mg ions is emphasized.