It measures 4cm (1.6in) across and would make $12 if sold for scrap silver today, but the "Amon Carter" Flowing Hair Silver Dollar is a record-breaker. It was bought by a private collector in 2013 for $10m (about £7m).

The astonishing price-tag is thanks to it ticking three all-important boxes - rarity, condition and cultural value. But perhaps the greatest selling point is who handled it.

Many experts believe it could be the first dollar ever to be struck. This means President George Washington himself would have personally inspected it.

What is known for sure is that it was one of 1,758 struck on 15 October 1794 at the fledgling Philadelphia Mint. Only about 130 survive today. It shows the figure of Liberty with long hair on one side, surrounded by 15 stars representing the 15 US states at that time. The reverse side features an eagle with its wings open and encircled by a wreath. In 1795 the Liberty flowing hair design was replaced by a draped bust design.

"A rare coin is one of a few thousand. We would class the 1794 coin as very or extremely rare," says Richard West from coin and medal auction house Baldwin's.

The coin was evaluated by the Professional Coin Grading Service in the US. It was judged to be 66 on a scale of one to 70, which is considered to be mint condition. and is the highest quality 1794 dollar known to exist.

"To think that a coin from the late 18th Century has survived in this state of preservation is remarkable," says Steve Roach from Coin World.

"It's also beautiful, with pleasing toning, a bold strike and its surfaces show reflectivity that strongly suggests that it was given special care and attention when it was struck."

"We think it was the first or among the first dollars struck because of the quality of the definition of the coin," says James Deeny, managing director at the London Mint Office, which is exhibiting the coin.

"In historic printing they used a hand-turned screw press where two moulds were brought together against a round metal planchet. Over time the repeated striking of the moulds meant they lost definition and also slipped a little out of alignment."

Experts say the coin's cultural significance is what contributes most to its hefty price tag.

"This coin represents the very beginning of what has become the world's most important currency," says Deeny.

After declaring independence in 1776, the United States relied mostly on foreign coins like the Spanish dollar for currency. In 1792 Congress established the Mint to create a new national coinage. The first copper cents were produced in 1793 and the first silver dollars were struck a year later.

The rise and rise of the $1 coin

1947: Sold for $1,250

1984: Sold for $264,000

1991: Sold for $506,000

2002: Sold for $2,000,000

2010: Sold for $7,850,000

2013: Sold for $10,000,000

"Benjamin Franklin said the ability of the US to create its own currency was an important step towards establishing itself as an independent nation. Coins are a badge of sovereignty."

The whereabouts of the $10m coin between 1794 and 1920 are a mystery, although Deeny says the lack of wear and tear prove it was kept in private hands and was never in circulation.

It first appeared in records in 1920 when it was bought by a businessman known as Colonel Green. It has since had 11 owners and has increased in price from $1,250 in 1947 to $10m in 2013. Deeny says the coin could go for more than $10m were it to be sold again and that the current owner, Bruce Morelan, would be interested in receiving offers.

Ultimately, West says the price will depend on the buyer.

"Coins that are unique are more difficult to value as you don't have other auctions to compare them to. The price really is determined by what people who can afford them are prepared to pay."

(BBC)