Amethyst shows distinct การเปลี่ยนสีคู่. 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 region of the spectrum from about 520 to 550 nm, but the spectrum of amethyst cannot called distinctive and is of little aid in identification. The amethyst is practically inert as regards any luminescence, but a blue glow has been seen with some amethyst when bathed in short-wave ultra-violet light.