The Australian 1930 Penny
This penny is recognized as being the world's most valuable copper coin. The catch is it must be one of the six proof 1930 Australian pennies that were produced. Never intended to be currency this penny was a show piece. The six were made with the purpose of a historical record for future generations to admire.
On todays market it would be expected for the sale price to be more than one million dollars for each 1930 Australian penny. When one was exhibited in 2007 it was described by Australian The Age newspaper as the "holy grail of coin collecting". About half of the six are in private hands while the rest are in public collections.
Though this penny may seem beyond the grasp of mortal collectors there are copies made in it's image. If you need to fill a gap in an Australian penny set then these replicas will serve your purposes well. eBay is a good source for such needs and you can see related 1930 Australian penny auctions listed below.