The yellow variety of quartz is called citrine. It may be natural, but in Brazil the natural citrine is very rare. What is done is to “burn” the amethysts. Eight hours to 400 degrees Celsius and it's done. However there is more to it. It'll depend on the amethyst. Not all amethyst turns yellow or into citrine. During the heat treatment, some amethyst will crack, others do not change colour, others become milky and such. There are so many tricks in the process and the amethyst need to be from the right mine.
But who, after all, has discovered that burnt amethyst turns yellow? A German immigrant, August Lamberts, worked with amethyst in Rio Grande do Sul. In 1883 he used some weak, pale or low value amethyst geodes to make a barbecue like the one I photographed near Ametista do Sul (RS) few years ago (see below). He roasted a barbecue for dinner, and in the morning discovered that the amethyst geodes were coloured with tints from yellow to red. Thus the Brazilian (Rio Grande) citrine industry was born.
-- Translated from original Portuguese text: Prof. Heinrich Theodor Frank’s post: https://www.facebook.com/permalink.php?story_fbid=…