One of the rare U.S. gold coins found in a cache discovered by a Northern California couple on their daily walk is the finest known coin of its type and valued around $1 million, experts said Wednesday.

An 1866 $20 coin printed without the “In God We Trust” motto -- the 1866-S No Motto Double Eagle -- is the highest quality of its kind, said David Hall, cofounder of Professional Coin Grading Services in Irvine, who recently authenticated the coins.

When the motto was added to the coin in 1866, some coins were still minted in San Francisco without the phrase, he said.

PHOTOS: California couple discovers cache of gold coins

At that time in San Francisco people were concerned with staying alive and feeding themselves, not preserving these coins for posterity, Hall said.

“It was the Wild West, so the coins weren’t saved. And this one survivor, it’s kind of a miraculous coin. It’s never been in circulation. It was saved from the day it was minted,” he said.

Had the couple attempted to clean the delicate surface of the coin they could have reduced the value to $7,000 or $8,000 in under a minute, said David McCarthy, senior numismatist for numismatic firm Kagin’s Inc., who evaluated the hoard.

The couple, identified by Kagin's only as "John" and "Mary," made the find during one of their daily walks on their property in California's gold country.

Eight of the rusty cans they dug up were filled with more than 1,400 rare and perfectly preserved U.S. gold coins dating from 1847 to 1894, according to Donald Kagin, president of Kagin’s.

The "Saddle Ridge hoard," comprised of nearly 1,400 $20 gold pieces, 50 $10 pieces and four $5 pieces, has a face value of more than $28,000 but could sell for more than $10 million, Kagin said. At least 13 of the coins are the finest known of their kind, he said.

“In terms of the condition and value of the coins, we as amateurs thought that the 1866-S No Motto $20 might be worth more than $5,000 or more -- we didn’t realize it was considerably better than the coin sitting in the Smithsonian!” John said in an online interview with Kagin.

The previous largest reported find of buried gold treasure in the U.S. had a face value of $4,500. It was discovered by construction workers in 1985 in Jackson, Tenn., and eventually sold for around $1 million.

A sampling of the collection will be displayed at Kagin’s booth during the American Numismatic Assn. National Money Show in Atlanta from Thursday through Saturday.

The couple said they planned to keep some of the coins and sell others on Amazon.com with the intention of donating part of the proceeds to charity. More importantly, they said, they will use the money to hold on to their home.  

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samantha.schaefer@latimes.com

Twitter: @Sam_Schaefer