Heat treated Amethyst is sold/marketed as Citrine, a rarer form of amber/yellow colored quartz. So a lot of the Citrine for sale today is basically fake. It takes a good bit of heat to make this change. I have never seen sunlight fade amethyst but other less stable minerals against sunlight, yes.
Amethyst in different color intensity is due to richer mineral ingredient at the time of crystallization, that is why you see color differences from different locales. All minerals are pure in color (clear/white) until other minerals invade the mixture and give it its color. In most cases its a metallic mineral introduced that gives a specific mineral its color (with glass making its the same). Oxidation can later occur underground (hydrothermally most of the time) and also change colors.
The FE iron ions that give Amethyst crystal its color can be diminished by UV rays according to studies.
Ok I found that photo. These amethyst bottles used to be owned by Scott Allen Bricker. 2nd and 4th bottles made by J T and A Hamilton glassworks in the late 20's- the other 3 with manganese are from unknown maker also from the late 20's maybe early 30's.
