Why symmetry elements involoving translation are not expressed in the external morphology of a crystal?

Translational symmetry is an aspect of symmetry. After you determine the crystal system, you use translation elements to determine the crystal cl or point group. Glide symmetry, for instance, means that there is translation along a mirror/glide plane, but this is at an atomic level. This type of symmetry is present in chain silicates (pyroxene, amphibole, etc). Screw axis symmetry is translation combined with rotation (180, 120, 90, or 60 degrees), similar to a spiral staircase. This is generally in framework silicates (like quartz). This combination of rotation around an axis and a translation parallel to the axis leave the crystal unchanged.