The theft happened on Thursday, the first day of the show, and according to owner Bob Campbell, the nearly 2-pound piece of metal is a precious collectors item.
"We've lost similar things, but the magnitude of this is a different level entirely," said Campbell, who's been coming to the Long Beach Expo for the last 40 years. "The person probably doesn't fully understand what he has."
The gold itself is worth tens of thousands of dollars, but the history of that specific nugget makes it even more valuable. It's linked to the Gold Rush in Northern California and was apparently discovered in the mid-1800s and preserved for more than 150 years in the same form as it was initially found.
"It's exactly what it looked like, and it hasn't been altered. This is what it would've looked like, I mean it might've been a little bit more shiny," said Campbell, who believes the thief was caught on surveillance video.
In footage provided to ABC affiliate KABC, you see a man wearing a black and gold shirt near the booth. It's a bit tough to see because it's a few tables back, but the man leans on the table then quickly walks away. You even see him glance back at the booth as he rushes off.
"You can see the guy reaching into the case. You can see him grabbing the nugget. You can see him putting it in his pocket, and we can see him put it away - it's a very good picture," said Campbell. "I know there's somebody out there that knows this guy."
Long Beach police officers were called to the expo, but haven't publicly said if the man in the video is a person of interest, saying only that they're investigating a theft.
Campbell's fear is he'll never see that man, or the precious gold nugget, ever again. He's worried it's already been melted down to be sold to the highest bidder.
"I'm willing to pay $10,000 to somebody who knows this guy," said Campbell. "Return the nugget, turn him in, get $10,000 - 10,000 reasons to do it."
Anyone with information is urged to contact police.