They call it green fire, emeralds buried deep in the earth that glitter in the light. They're some of the priciest precious stones on the planet. And tonight, an expert gem hunter takes us to the... See More
They call it green fire, emeralds buried deep in the earth that glitter in the light. They're some of the priciest precious stones on the planet. And tonight, an expert gem hunter takes us to the hidden mines of colombia where few dare to go. And just a single stone could transform a meiner's life forever. If they can get it. Here's our series "off the map." Deep underground in the mountains of colombia, we're on hunt for one of the world's most precious stones. We're about one football field below. Reporter: A green rock so dazzling, so valuable these miners are willing to risk everything for a chance to find just one. Hear that? That's the jackhammer. They lead me down a dark, wet maze of tunnels, through steal ga steel gates meant to keep intruders out. Do you hear that dynamite? Let's get out of here. They're going to blast over there. Let's go, let's go. . Reporter: It's a wild frontier ruled by paramilitaries, thieves, and occasionally angry mobs. Everybody is furious. Reporter: But for those hardy enough to tough it out, here's the prize. This one is from there? Whoa, these are huge. Beautiful emeralds that could make them rich beyond their wildest dreams. Wow. Our journey begins with the breathtaking life over colombia's fresh fields of coffee and sugar cane. But the beauty is deceiving. Many of those winding roads are ruled by gorilla fiuerrilla fighters. It will be too dangerous. We're flying over these beautiful, beautiful lands. It's a green fire obsession. Reporter: He spent a lifetime chasing that obsession. An he's within of colombia's top gem hunters. It's an incredible feeling. It's very rare. Reporter: For 40 year, he's tracked down the wofrld's most expensive emeralds. Why is there so much security here? Double locks on all the doors? It's worth a lot of money. Reporter: They'll play top dollar for private collectors and big museums. He's so well known in bogota's emerald districts, sellers swarm us, hoping to catch his eye. 500 for all of them. Reporter: Is that a good deal? No, not at all. Reporter: It takes a special stone and that's why he's agreed to take us directly to the source. The muzo mines, home to the richest emerald deposits in the world. That's river is called the miner's river. Tooth right is the town. And to the left are the mines. All these are mines. Reporter: Turf wars here left thousands dead. And while the violence is nothing like it was, the threat of kidnapping, even murder is still very real. On the ground, we're greeted by the mine's manager. He tells us we only have a few hours to explore. He can't guarantee our safety once the local gangs and militias find out we're here. We're going to a mine that's a very isolated road. Reporter: This is a land of desperation for so many people? Yes, complete survival. And let me tell you, people here will do anything to get themselves one way into the tunnels. And they will work for no salary. To be a miner, it's hard, hard. I mean, just to work in a mine is -- it's horrendous. Reporter: And they're the lucky ones. Because some only get this far, digging through the leftovers. So they're not employed by the company, but they're allowed to do this? Whatever they find is theirs. They don't share it with the company. Reporter: Then we find a more desperate scene. Women and even children sifting through scraps in the river. The mining company gives them these bags as a good will gesture. But it only stirs more resentment. Have all these people been waiting until 2:00 in the morning. Nada? You can see, all of these people are just running down the hill. They're only allowed to have one of these bags filled with the dirt that came out of the pit. Their dream is to go down, wash it out of the water with the river and find some emeralds inside. They're still poring through. Have you ever found an emerald in your life? No, just leftover. Still proof that the best emeralds are deep underground. The company isn't happy we're here. They don't want us to talk to these people anymore. They want us to get out and so we're going to get out of here. As we suit up to g into the mines, they give us after safety briefing that's unsettling. Reporter: If there's an explosion, there's no danger the tunnel will collapse around us? Reporter: Okay, guys, let's go. Pretty tight. This is the oxygen so we can breathe. So we have oar going to now go downward on this elevator. When we get down to the bottom, we're going to then cut to a horizontal tunnel and continue on that. It will be interesting. The noise is deafening. But nothing compared to what I'm about to experience. The jackhammers stop and behind the rock, you heard that dynamite. We meet a 67-year-old miner who's been working these tunnels for 40 years. How many emeralds do you think you found? Few, but he's been able to survive. Would you tell your friends or children to have this job? No, never. Reporter: Is this the last place they found emeralds the middle of last year? A year snoog -- ago? They told me they haven't found emeralds since mid last year. You're kidding me? Those guys back there told us one of their colleagues finally just gave it up. He can't take it anymore. Even when they do find emeralds, sometimes the company never finds out. How do they smuggle it out of here? They put it in their pocket. Sometimes they swallow them. When you find some, do you put it in your pocket and walk away with it? The guy in the blue helmet isn't laughing. Because he' sent here by the mining company to keep an eye on everyone to make sure we're not stealing anything. Whoa, these are huge. Wow. Reporter: But above ground, proof that even the best security measures don't always work. All of these guys have been mining for them? Yeah, everybody. Everybody here. Reporter: How much do you think all of this is worth? Maybe around $5,000. Reporter: Do you think that's about right? No. Reporter: He's just hoping we're suckers and pay $5,000 for this. But now our time is almost up. We need to get back to that helicopter before anyone gives us trouble. Let's go. Reporter: Most beautiful valley I've been to for a long time. So we must leave this place empty happeneded. But as we fly over the desperate prospectors still sifting in the river below, we get this reminder from the veteran of these hills. They feel the emerald is going to satisfy all their needs. They'll hit the pocket and wow. Reporter: For "nightline," I'm bob woodruff.
This transcript has been automatically generated and may not be 100% accurate.