I also have a piece in my personal collection that’s a synth. ametrine; it’s half amethyst and half citrine. Being able to separate natural from synth. amethyst was a problem, but the people in our GTL found the answer. It has to do with how they grow. I’ve attached an article that was published in 1986 in our Gems & Gemology journal. It’s very technical, but I thought you might want to see it. The three authors were considered some of the top experts in the world on gemstones at that time.

http://www.gia.edu/gems-gemology/fal...crowningshield

Dr. Karl Schmetzer recently showed how near-flawless to flawless natural amethyst could be separated from synthetic amethyst on the basis of the presence of Brazil twinning in the natural stones. Whereas Dr. Schmetzer's procedure required a special apparatus, the authors have determined that a standard gemological polariscope is more than adequate to make the separation in most cases. Although some synthetic amethyst does show evidence of twinning, in the synthetic stones examined thus far it has taken a form that is distinctly different from the Brazil twinning seen in most natural amethysts. The presence of certain inclusions as well as the nature of the color zoning seen in natural versus synthetic amethysts is of primary use in making a separation. However, where there are no inclusions or color zoning, the presence of Brazil twinning in the natural amethyst will usually make the distinction.