An incredibly rare Roman coin discovered in Acle has been donated to Norwich Castle Museum.

The coin - only the second of its kind known in the world - was unearthed by Dave Clarke during the Springfield archaeology dig last summer.

Acle Parish Council has sent the ancient artefact to Norwich where it may go on display and will be used by experts to identify and date other coins.

The coin dates from AD 312 when Emperor Constantine I ruled the Roman world. The only other example was found in the 18th Century and is on show in Lisbon.

One side shows the bust of Constantine the Great and the reverse pictures him seated in a chair holding a sceptre. It was the tail-side which grabbed the attention of Dr Adrian Marsden, coin specialist for Norfolk County Council's Historic Environment Service, while he was looking through finds at a meeting of the East Norfolk Metal Detecting Society in Hemsby.

The picture reveals the exact office Constantine held at the time; the office of the consultate, dating the coin to AD 312. It also carries a mark revealing it was minted in London.

'Very few coins from that time are dated,' said Dr Marsden.

'We have coins from that era but now we can tie all those with the same mint mark to this date. It will help us identity other finds.

'It's only the second of its kind. The other is held in a museum in Lisbon which makes this find even more interesting.'

The coin will go on display in the recent finds case at Norwich Castle Museum.

A photograph is on show in the Roman finds cabinet at the Acle Recreation and Social Club.

Another Roman coin found by Dave Clarke at the dig was a copper alloy coin known as a sestertius that depicts Roman emperor usurper Clodius Albinis and dates from AD 194-5. It has been put in the Springfield dig display unit in Acle Library.