A two-and-a-half kilogram gold nugget worth almost half a million dollars has been unearthed using a metal detector in the US.

While the name of the lucky digger is being kept a secret, Bob Van Camp, a resident of the US town of Paradise, was happy to pose with the California find in video promoting its unveiling later this month.

The nugget is almost pure gold and has been valued at up to $460,000, according to Mr Van Camp.

The metal detector dealer said his client, who he calls Mr Smith, would not reveal to him the exact location of the July discovery, just that it was found in the foothills of Butte County public land that was "worked" during the original Gold Rush.

He points out that neither he nor Mr Smith has the nugget in their possession.

Rather, it is being held under lock and key at a nearby auction house until the unveiling ceremony.