Where can I find authentic amber and garnet jewelry without being ripped off?
Prague is still relatively cheap, so it's unlikely you will truly get ripped off. That said, Prague is full of souvenir stores that cater mostly to tourists. I don't know how "authentic" any of it is.
But if you really want inexpensive amber... go to Poland or Lithuania.
I'm going to be in Prague in a few weeks and I found this website through my research: http://www.czechtourism.com/index.php?show=003005022005&lang=3
quoted from website:
"Garnets! These red gems are iconically Czech, and signs advertising “Czech Garnet Jewelry” abound in downtown Prague. You should know, however, that only one company in the entire country (Granát Turnov) mines and sets the garnets, so whatever isn’t produced by them is either an antique or a fake. If you are buying a genuine piece of Czech garnet jewelry, you should automatically receive a certificate of authenticity. True, Granát has its own factory stores on Dlouhá and Panská streets, but the largest selection of garnet jewelry in Prague is carried by a small shop located at the very beginning of Celetná street, just off of old Town Square."