Buying gems and jewelry in your own backyard is difficult enough without knowing a reputable jeweler. Going abroad and buying compounds your problem three-fold.
To begin with, what if after returning home you find out what you purchased is not worth anywhere near what you paid for it? Can you return it for credit or exchange? In most cases, that's not possible or practical. The customs problem enters into the picture. Is there going to be duty on it going each way?
What if the gem has been treated or enhanced in some manner to improve its appearance and they haven't disclosed that it is not natural? You still have the same problem about returning it. They may give you a certificate authenticating origin, being natural and untreated, etc., but will they honor the certificate? Good luck! Is there a Better Business Bureau or Consumers Affairs Agency there? I doubt it. One possible safeguard is paying with a check or credit card, provided you get home in time to challenge it.
Unless a very reliable source has recommended someone beyond reproach, I would advise your purchases to be monetarily insignificant. An inexpensive curio, perhaps, to give you fond memories of the trip is what I would advise.
Bon Voyage!