There's nothing more valuable than a personal recommendation from friends you trust when you're looking for a new vendor/restaurant/doctor/dominatrix. That's what my wife and I got (not the dominatrix, the recommendation), and it was for Gaspar Jewelers. Gaspar had created custom rings for my friends' wedding and they were totally satisfied with the results.
After losing my ring in a Mexican riptide, I had to replace it. But I wanted a custom design. Gaspar worked with us creatively on the design, he worked with us within the budget we had alotted, and he was a pleasure to be around.
Gaspar created a simpler albeit custom ring for my wife and she was so impressed that she had him do about 30 repairs on some old jewelry. When a couple of the pieces needed further adjustments, he stood behind his work.
He speaks both English and Spanish, his prices are very competitive, even in some cases cheaper than street vendors.
The only thing to be aware of is that if you've got a constrained budget, and you're making a custom piece, it may not happen quickly. I was fine with this, particularly because it allowed us to go back and forth with design ideas, and I was able to approve the process at each step. At one point Gaspar showed me the wax mold proof of the ring, which was incredibly helpful and I saw how the worksmanship was even better than I had expected.
Park in Pershing Square. Gaspar Jewelers validates with purchase.
Pros: friendly, inexpensive, high-quality work
Cons: takes some time on custom design if you get low price