We all have one, that bottle, jar, dish or piggy bank full of loose change we hope one day will add up to a whole lot more. But that day may well have come and gone and you didn't even realise it.
At least once in recent years, as metal prices surged, the cost of making a humble coin exceeded its denomination. That is, the metals required to mint a HK$1 coin cost more than the value of a HK$1 coin.
With some of them forgotten at home, many lost and others damaged, the number of coins in circulation has dropped by 280 million to 5.25 billion since the Hong Kong government stopped making new coins in 1998. Over the same period, metal prices have rocketed and it could cost the Monetary Authority a fortune to make new coins now.
According to the regulator, HK$1, HK$2 and HK$5 coins are made of cupronickel, a copper alloy usually containing 75 per cent copper and 25 per cent nickel.
In 1998, copper cost the equivalent of about 1 HK cent per gram and nickel about 5 HK cents on the London Metal Exchange.
But nine years later, costs have surged to 6 HK cents per gram for copper and 41 HK cents for nickel.
This means the cost of making a HK$1 coin, excluding labour costs, was 15 HK cents in 1998. But in 2007, when nickel prices peaked, the cost reached HK$1.07 - more than the coin's face value.
Now the cost is around 66 cents, still six times as much as in 1998.
Surging metal prices presented a headache for many countries, the president of the Numismatic Society, Ma Tak-wo, said.
'The turbulent nickel market is a pain for minters,' he said. Many regions, including Hong Kong, had sought to use steel as an alternative.
In the 80s, all the local cent coins were made of nickel brass, a combination of copper, zinc and nickel, but since 1993, brass-plated steel had been used instead for 10- and 50-cent coins, Ma said.
On the other hand, 20-cent coins are still made of nickel brass. A rough calculation by the Post showed the cost of making a 20 cent coin out of nickel brass was about 15 HK cents, while making it out of steel would cost about seven-tenths of one cent.
'If the government wants to reduce the cost in making coins, it can change the metal components or reduce their weight,' Ma suggested.
Changing the weight of coins was unusual, but not impossible, and usually took place when there was a complete redesign of a coin.
However, as soft drink and snack machines recognised coins of different values by their weight, if the weight of coins was changed, people had to change the setting of vendor machines too, he said.
The Elizabeth II coin, minted from the 1970s to the 1990s, had a wavy edge and weighed 2.59g. While a bauhinia had replaced the Queen's face on coins after the handover, their weight was the same, Ma said.
The rise in the use of 'electronic money' had cut public demand for coins, senior economist at Hang Seng Bank Irina Fan Yuen-yee said.
'Octopus cards and credit cards can be used at stores. People do not use physical money that often,' she said. Coins of smaller denominations could be phased out over coming years, she said.
Dr Mo Pak-hung, an economist at Baptist University, said fast development in Asia ruled out chances for a big drop in metal prices, but coin making would not be a trouble as long as governments explored the use of different materials.
There had been no minting of new coins since 1999 because the demand had been stable, a Monetary Authority spokesman said. It would continue to monitor the trend to see if it was necessary to mint new coins.