A US silver dollar worth more than $10m has gone on display in London. But what makes this coin the most valuable in the world?

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.

"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."

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.

To read the complete article, see:
Why is this coin worth $10m? (www.bbc.com/news/magazine-35821154)