Sometimes the inclusions extend out to and damage crystal faces. Sometimes the faces are imperfect anyway. These faults tend to impair particular halos. For example, column crystal end faces are most often imperfect thus blocking rays which would otherwise form the 46 halo, infralateral and supralateral arcs. Precise crystal alignment is needed for many halos.
Some halos are so large and their colours so dispersed that they are inherently faint.
The effects of imperfect alignment. At top right clear and distinct upper tangent and Parry arcs are formed by column crystals with tilts of only 0.1 std dev. As the crystal alignment becomes poorer the Parry arc becomes less and less distinct.