Crystal canuck wrote "The average layman might have a hard time telling apart man-made and natural, but any gemologist that is half decent can tell the difference."
You provided this link
https://www.gia.edu/gem-treatment which if you look at the 8th paragraph, emphasizes the issues of being able to tell man-made from natural. It is not as easy as you claim.
If you get gems that have been certified by certain labs, they can tell you what the gemstone is but in many cases can not tell if it was man-made or natural.
About 15 years ago I visited the largest market for colored stones. Even to this day I wonder...... if the blue star Sapphire a woman presented to me was real. All I can say it was gorgeous!