A US 1942 experimental glass cent sold for $70,500 at Heritage Auctions on January 5.

The lot was one of the stars of the annual Florida United Numismatists (FUN) convention, more than doubling its $30,000 estimate.

The US Mint experimented with unconventional materials during the second world war

The coin came about due to the rarity of copper during the war.

The coin's consignor, Roger Burdette, an expert in experimental US coins from this era, said: "Wartime scarcity of copper required the U.S. mint replace copper for the one cent coin.

"Plastics fabricators, particularly those who made buttons, began to experiment with pieces the size of a cent but the Blue Ridge Glass Company of Kingsport, Tennessee, requested an opportunity to experiment with glass in late 1942."

The cent never entered circulation, as it proved very difficult to get the die and the glass to the requisite temperature.

This meant the glass could only be imprinted with a faint impression of the die, rendering it useless as currency.

Mark Borckardt, Heritage Auctions' senior numismatist, said: "The present 1942 glass experimental piece is the only intact example discovered in nearly 75 years since the experiments.

"Although glass was never used for emergency U.S. coinage, this piece represents a unique artifact of the ingenuity and determination of Mint officials and private industry."