Amethyst shows distinct dichroism. the twin colours being bluish-violet and reddish-violet for the ordinary and extraordinary rays respectively. However heat-treated the brown amethyst does not show dichroism. and thus treated stones may be detected from those of natural brown or yellow by their lack of dichroism. Under the Chelsea colour filter amethysts appear a reddish colour the stronger the more pronounced the body colour of the stone. There is a wide absorption of varying intensity in the yellow-green from about 520 to 550 nm. but the spectrum of region of the spectrum cannot called distinctive and is of little aid in identification. The amethyst is practically inert as luminescence, but a blue glow has been seen with some amethyst regards any when bathed in short-wave ultra-violet light. Inclusions 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 (Figure 10.8) which is ascribed to twinning or to the partial healing of separations rhombohedral occurring along rhombohedral faces. Very few amethysts are free from the planes of the major ari coloration with angular zones of colour, and even in the heat-treated rown stones this zonal colour marking is seen. Cacoxenite crystals are often seen in amethyst.