Another really good jeweler is Abraxis Antiques- 1092 Defensa, across from Plaza Dorrego in San Telmo. They have really high quality estate (vintage) jewelry. We have bought several amazing pieces from them, prices are very good for the quality. Not a huge selection, but very good stuff, an old family that has been in the jewelry business a long time. Pearls, diamonds, rubies and emeralds, deco and art noveau pieces. Things that would be hard to find at double or triple the price in the USA.
Libertad near Corrientes has several dozen jewelers, and there are many there that have good quality pieces as well. Generally prices are based on weight, to begin with, although obviously diamonds are different.