The Miami Metromall, located in downtown Miami, Florida, is probably one of the most interesting little semi-hidden places in the city.
The Metromall is located at 1 Northeast 1st Street, just across the street and down from the courthouse. The odd, fascinating little labrynth of the Metromall contains several dozen shops selling jewelry and jewelry-making supplies.
In the various stores around the Metromall, you can find almost any precious or semi-precious stone imaginable, beads, findings, precious metals and the tools and equipment needed to make them into something beautiful, and jewelry of all kinds. There are shops specializing in repairs of jewelry and watches, and at least one on the basement level that does castings, so you can have your own wax creation cast in gold or silver.
Some of the stores in the Metromall are wholesale dealers only, but most are open to the public. One of the coolest stores there (I believe it's on the second floor) offers huge strings of polished gemstone beads in almost every shape and size imaginable, and has ridiculously low prices... but has a $100 minimum purchase.
The Metromall is a fascinating place to visit, but the building is dangerously showing its age. Be very careful on the escalators, as they tend to lurch forwards, and the moving handrails often stop moving completely independent of the steps.