The behavior of anisotropically etched crystalline materials is studied at the geometrical level. The crystal shape is represented with a polyhedral boundary description in which the various faces advance with rates solely dependent on their orientations. Particular attention is paid to the situations under which new truncation or bevel faces that were not previously present appear at vertices or edges of the crystal. A new method of analysis based on convex hulls over the extrema of the inverse of the etch-rate polar diagram (slowness diagram) is presented which can handle corners of arbitrary complexity. Results are presented that were obtained with a prototype of a 3-dimensional etching simulator based on this representation.