From "The Timeless Mystique of Emeralds," by Fred Ward in the July National Geographic:

Violence over emeralds began with the conquistadors in Colombia and has never stopped. I had to postpone my first visit there last year. Two weeks before I was to arrive, my host, one of the three principals at {the emerald mine of} Muzo ... was gunned down at his ranch along with 17 bodyguards. Later, when I resumed my Colombian plans, I was advised that all ground transportation is risky. Driving to a mine in a car is dangerous because local banditos know passengers carry cash to buy emeralds. Driving back to Bogota is dangerous because passengers carry newly purchased stones. ... Once last year the mine organized a 50-jeep convoy to take in supplies, and even that was attacked, with eight workers killed.

More than a dozen Colombians I interviewed for this article have since been murdered. When the Bogota police gave almost a month's warning before an arms check in a four-block downtown quadrant near the emerald market, they still confiscated over 600 weapons. The safest assumption for a visitor is that everyone but you has a gun. All the dealers I saw had pistols strapped to their ankles, on their belts, in their handbags, in cars, on airplanes and on tables. A visiting group of Israeli gem traders was offered a kilogram of rough emeralds for six Galil automatic rifles. ...

For safety's sake I had opted to commute to Muzo by helicopter. There, during an early breakfast with 12 workers, I counted 11 pistols, either holstered or stuck into belts. I assumed the one unarmed man just wasn't fully dressed yet.