The nugget, which weighs nearly 100 ounces, was found in February near the town of Washington.
The current price of gold would make that chunk worth about $135,000, but one expert, Fred Holabird, told The Sacramento Bee he thinks it's the largest remaining California nugget and could command $400,000.
The so-called Washington Nugget came from an old riverbed, where 150 years ago hydraulic miners took all the gold they could, he said.
"This ended up being a little piece that got missed," said Holabird, a mining geologist who will sell the nugget at his auction house. "This was a chance thing."
An unidentified property owner who found the nugget brought it to Holabird for an assessment.
"I tried to find out if any of the big ones still existed, and they don't," he said.
He believes nuggets of that size from California have all been melted into bullion or coins.
"It truly is not one in a million," Holabird said. "It's one in a billion."
Read more at The Sacramento Bee.