The Professional Coin Grading Service - PCGS - recently compiled a list of the most valuable US coins in 2011.  Following on from our article on the 'Million Dollar Coin Club', this week we provide a run-down of the top five rarest and most expensive pieces according to the PCGS.

Since 1986, the service has graded over 21 million coins with a combined value of more than $20bn.  Their recent report stated that rare US coins have increased in value by between 20 and 50% in the last year.

Below are the top five most valuable US coins in 2011 - representing five good reasons why you should consider coins as an alternative investment . . .

The 1849 $20 'Double Eagle' Liberty coin

This historic coin, currently housed in the Smithsonian's National Museum of America History, was the first $20 gold piece struck at the United States Mint in Philadelphia.  Worth $15m a year ago, the value has leapt up to an amazing $20m in 2011.

The 1907 Indian Head $20 coin

Designed by sculptor Augustus Saint-Gaudens, this rare and beautiful gold pattern piece combines a $10 Indian Head obverse design with a $20 reverse design.  Now part of a private collection, it is estimated to be worth $15m.

The 1877 $50 gold pattern J-1546 coin

This famous 'Half Union' gold piece is the largest coin ever struck by the United States Mint, weighing 2.5 ounces.  Only two were ever created - the other with a different design - and are kept in the Smithsonian.  It is worth approximately $15m, $5m more than last year.

The 1907 Double Thick Extremely High Relief $20 coin

Two of these coins are held by the Smithsonian, worth $8.5m each.  The brainchild of Teddy Roosevelt and again designed by Augustus Saint-Gaudens, they are often considered the most beautiful coins ever made, picturing Liberty and an Eagle in magnificent detail.

The 1794 Silver Dollar coin

What is believed to be the very first silver dollar struck by the United States Mint, this historic coin is worth $7.85m - $2.85m more than last year.

The 1804 Class I 'Original' Silver Dollar coin

Only 15 of these were made and this piece, held in a private collection, is the most pristine example.  In 1836, it was presented as a gift from President Andrew Jackson to the Sultan of Muscat.  It is now worth $7.5m.

The continuing popularity of coins is a good sign for numismatic investors; and you don't have to have a museum or a million to own a rare piece.  We recently reported on a London Coins auction in early March, which offers plenty of excellent items for only four figures.