'It's like finding a Picasso at a garage sale': 'Fake' coin from California Gold Rush is actually REAL and worth MILLIONS and its owner had no idea.

An incredibly rare coin from the California Gold Rush is worth millions - and its owner didn't have a clue.

The San Francisco Mint produced fewer than 300 special edition $5 coins, and it was thought that only three survived.

The anonymous owner of the discovered fourth coin believed the piece to be fake, as did multiple coin dealers

However, the Numismatic Guaranty Corporation (NGC), the world's largest rare coin authentication company, proved it to be real by comparing it to the three known coins.

The 164-year-old treasure is now worth millions of dollars, according to the NGC.

'It's like finding an original Picasso at a garage sale. It's the discovery of a lifetime,' said Mark Salzberg, NGC Chairman to PR Newswire.

'The owner of the coin is a life-long New England resident who wants to remain anonymous. He was stunned when we informed him that it is a genuine, multi-million dollar rare coin,' Salzberg said.

'He had shown it to a few collectors and dealers at a recent coin show, but everybody said they thought it was a fake because until now there were only three genuine surviving 1854 San Francisco Mint $5 gold pieces known,' he said.

One of the three known coins was stolen from the Du Pont family in 1967 and never recovered. After it was confirmed this was not the missing coin, it was then compared to high-resolution photos from the Smithsonian, where one of the coins is kept.

'We look for common things that you’ll see between the coins,' Richard Montgomery, president of NGC, told Gizmodo.

'You’ll see the four in the digit is slightly attenuated or not as high in relief as the regular part of the four. We noticed that was exactly the same as the Smithsonian piece.'