The double thickness silver artefact is French and dated to 1322-28
A Shropshire woman is the first in the UK to be convicted of failing to report treasure, say West Mercia police.
Kate Harding, 23, of The Hawthorns in Ludlow, presented a 14th Century artefact to Ludlow Museum in January last year.
After it was identified by experts from The British Museum she was told she had to report the find to the coroner.
But she did not and on Thursday admitted breaching the Treasure Act, at Ludlow Magistrates' Court.
She was given a three month conditional discharge, ordered to hand over the artefact and to pay £25 in costs.
Dr Michael Lewis of the British Museum confirmed the case was the first to have resulted in prosecution.
He said: "This is a landmark case and it sends a clear message to those who fail to report Treasure.
"However, we are delighted that the artefact, which has great historical significance, has now been handed over under the instruction of the magistrates."
A similar item was discovered in West Clandon, Surrey in 2007, and bought by the British Museum for £1,800.
But Ms Harding may not receive anything, as the Treasure Valuation Committee has the power to cut or cancel any payment for "wrongdoing".
Under the 1996 Treasure Act, anyone finding an ancient artefact must report the find to the coroner within 14 days.