Keith Andreae unearthed a rare half-crown issued during the Irish Rebellion in 1642
At first he thought he had found a small piece of silver bullion, but Mr Andreae handed it to Surrey finds liaison officer David Williams for a closer look.
It turned out the find was an exceptionally rare Irish half-crown minted during the Irish Rebellion of 1642 as an emergency currency by the Lords Justices of Dublin.
The coin is an irregular polygon, stamped on both sides with the weight of a half-crown in pennyweight and grains (9dwt, 6gr).
During the turbulent reign of Charles I, emergency silver coins were issued in places such as Newark and Scarborough. The latter issue was known as Scarborough Besieged coinage.
The Irish Rebellion coins are sometimes known as Inchiquin Money after Lord Inchiquin, who commanded the Protestant forces in Munster. They are made from cut pieces of flattened plate.
Mr Andreae, a retired aquarium shop owner, was out with colleagues from a metal detecting club when he picked up a signal from under the earth.
A British Museum spokesman said: "This is a find of note."
Mr Andreae told the Advertiser he has been allowed to keep the coin as it does not fall under the treasure trove laws, being just a single coin find.
The coin has been added to the British Museum's online archive, but its finder is yet to decide whether to sell it in an auction or to a museum, or whether to keep it.
Mr Andreae has only been metal detecting for about five years but has discovered a number of silver medieval coins and Roman currency.
He is a member of the Weald and Downland Metal Detecting Club that meets in Reigate, and a member of another club whose digs are mainly carried out in Wiltshire, Oxfordshire and Buckinghamshire.
The coin can be seen on the website at
www.finds.org.uk by clicking on "database" and typing in the find number SUR – 981DD2.
SCARCE: The Irish Rebellion coin found by Keith Andreae
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