I have cut synthetic amethyst, from a Russian dealer at the Tucson show, and the stuff is fabulous, totally flawless and gorgeous color, maybe a wee bit too dark, limiting the size of cut stones (think Welch’s grape jelly). As far as the synthetic amethyst goes, there really isn’t a test that will distinguish the natural from the synthetic, but the clarity and color of the synthetic are a tip off.
From the same dealer, I purchased a large synthetic emerald that I haven’t cut yet. Same goes here, gorgeous color and clarity, but these hydrothermally grown emeralds can be detected by flux inclusions under a microscope. The tip off to the eye is the dark color and flawlessness. If this stuff was natural, there is no way on earth I could afford it.
BTW,it was about 10 years ago I purchased these, the amethyst crystal was 1 1/4″ x 1 1/2″ x 7″ and cost $80.
The emerald is about 1 3/4″ x 1/2″, and was $120. Both have a thin seed crystal running through the middle.
As stated above, disclosure is required. But if you want to cut a large flawless stone without breaking the bank, you should give it a try.