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The Gold penny of Henry III is one of the rarest and most valuable of all English gold coins. Lost completely for centuries; between three and eight specimens are now known.
History of the Gold Penny
For a long time it was thought that the first English gold coin struck since Anglo Saxon times was the gold florin of Edward III, made in 1344. However, documentation came to light in the 1700's that in the reign of Henry III a gold penny was struck. This gold penny, made in 1257, was the first English hammered gold coin struck since before the Norman conquest - and came fairly close to being lost forever to the mists of time.
Gold Penny of King Henry III (1257)
One of only a handful of known genuine examples!
According to 18th century historians (Maitland, History of London, 1739, Vol.II, p.111)  , the dusty account of the gold penny had recently been discovered, having been preserved in the archives of the city of London. Maitland wrote: "It may, perhaps, appear strange that Henry, in the height of his distress for want of money, should be the first prince that ever struck gold in England. The piece he caused to be struck was of pure gold, and weighed two sterlings; it was to pass in the usual proportion of gold to silver at that time, for twenty sterlings or pennies in silver."
Henry's gold coin had been unpopular: Less than three months after introduction of the coin, the city petitioned against it. Documentation shows that the exchange value of the coin was raised from twenty to twenty-four silver pennies - and then another proclamation was made to the effect that no-one was obliged to take the coin, and that anyone might bring it in to the royal exchange and receive the current rate in silver, minus a halfpenny transaction fee.
So it appears that within a year, the gold penny was gone. Its demise was caused by discrepancy in the gold-silver ratio, meaning that people were unwilling to trade them. There is also evidence that gold coins were rather unnecessary at that time, the items being traded by most people being within reach of purchase with silver coins. Perhaps then, as is sometimes the case now, people were simply resistant to change, preferring what they already knew.
Design of the Gold Penny
Design: On the obverse we see King Henry III seated on a throne, crowned, and wearing his royal robes. In his right hand is a scepter, which passes under his arm. In his left, an orb or monde surmounted by a cross of four pellets. Under his feet there is a mosaic design. The inscription reads HENRICUS REX III. The reverse of the coin features a long cross, each limb bottoné, extending to the edges of the coin. A beaded circle divides the outer legend from the inner section, in which, in each of the interstices of the cross is found a five-petalled rose set between three small pellets. The legend states the minter and the town where the coin was made - "WILLEM ON LVND".  Three characters form the legend appear in each quarter of the coin - and it takes careful examination of the coin to decode the legend as it appears "WIL LEM ONL VND". Some examples read "LVNDEN" or "LVNDE"  instead of LVND. Willem was the King's Goldsmith, William of Gloucester  or William Fitz Otho .
The design of the coin, featuring the king, enthroned, in his royal robes, technically makes the piece a Ryal - and although it is often thought that the later English Gold Ryal was an imitation of French coins, the gold penny in fact preceded the Ryal of Phillip le Belle of France, whose reign commenced in 1285.  It appears that the design was inspired by coins of Edward the Confessor (1042-1066), whom Henry is thought to have idealised. 
Of the existing coins, it is known that varying dies were used as the designs of the existing coins differ slightly.
How Many Henry III Gold Pennies Exist?
The King's Goldsmith, William of Gloucester, is recorded as having made a delivery of 466 marks worth of the new coins (37,280 gold pennies) on August 27th, 1257. A further 190 marks (15,200 gold pennies) were reported delivered on October 1258. 
It is now assumed that all the gold pennies that were in circulation were simply either melted down for profit by individuals or traded in to the royal exchange for more valuable / popular silver coins.
Within a year or so, the gold pennies were gone. For centuries the coin was entirely forgotten - however with any circulation of coins a few are lost - perhaps falling from pockets or being buried in treasure hoards. And so, since the 18th century a few gold pennies have come to light.
It is reported that by 1806, three gold pennies were known - one of which was owned by Mr. Hodsol and another by Mr. Solly. 
There are discrepancies as to how many of these coins are actually in existence. Some sources state that there are three known specimens; however the Fitzwilliam museum reports eight and lists some provenance for several of the coins.  Six were apparently recorded by Sir John Evans in his The First Gold Coins of England (Numismatic Chronicle, 1900) and six were mentioned in the Numismatic Circular of 1903.  It appears that eight is close to or is indeed the actual number in existence.
Two are in the British museum. One sold at Captain Marchison's sale of June 1864.  Spink sold one by auction in July 1996 for a then-record £149,500.
It is of course quite likely that more of these coins exist - however finding them would truly be the equivalent of finding a needle in a haystack - or even in a shire of haystacks. How many of the 52,480 (?) gold pennies survived the melting pot? It is of course an unanswerable question. As for finding one - there is simply no way to know where to begin, other than to pick up a metal detector and get out there. Even then, the chances of finding one are very slim and it is worth reflecting that of the many dedicated metal detectorists combing the English fields and lanes, all with the exception of one or two have done a lifetime's work without finding such a coin.
Weight and Fineness of the Henry III Gold Penny
The gold penny is recorded as weighing 45 troy grains, and is made of fine gold. Prior to 1526, the "pound sterling" was based on the tower pound, which was 15/16 of a pound or 5,400 grains. So there would have been 120 of these gold coins made from a tower pound of gold. The fineness (purity) of the coin is described as being "23 carats 3 1/2 grains of fine gold, and 1/2 carat gr. alloy."  This needs a little explanation - in old times the carat was equivalent to four grains.   So we can say that the purity was 95.5 grains gold to half a grain alloy - equivalent to 191/192 purity; or 0.99479 - very close to the 995 standard of modern times.