If you've passed the intersection of Carrera Septima and Jimenez Ave., you probably noticed the crowd of men standing on the sidewalk bunched into little groups.

No, they aren't unemployed waiting for casual labor, or protesters waiting to march on Plaza Bolivar.

Rather, these guys are emerald traders and this patch of sidewalk is Bogotá's traditional open air emerald trading floor.

They aren't as famous as Colombia's other traditional exports, coffee and flowers. But Colombia is the world's emerald powerhouse, producing more than half of the world's supply, and particularly high-quality emeralds. Most of Colombia's emerald mines are in Boyaca Department, just a few hours from Bogotá.

Want to buy an emerald? You can find many displayed in windows of the gem shops on the streets of La Candelaria's emerald district. You can also try your luck and bargaining skills in the sidewalk emerald market, where I've seen prices drop by half or even two thirds from the initial asking price. But it's important to know a bit about valuing emeralds: a stone's worth is determined by its size, shape, depth of the greenness and the lack of internal flaws. That's why you'll see the traders holding stones up to the light to study their internal quality.

And, of course, be sure to be able to distinguish stone from green glass.

I've heard of travelers paying for their vacations by bringing a few emeralds home for resale. But even if you don't know how to deal in emeralds, you can pick up a few cheap ones on the sidewalk for a set of earrings or a ring for that special somebody.