The inclusions which may be seen in violet quartz are most commonly feathers made up of negative cavities. A quite common imperfection in amethyst is a mark like a thumb-print or tiger stripes which is ascribed to rhombohedra twinning or to the partial healing of separations occurring along the planes of the major rhombohedral faces. Very few amethysts are free from parfi-coloration with angular zones of colour, and even in the heat-treated brown stones this zonal colour marking is seen. Cacoxenite crystals are often seen in amethyst.