METAL detector enthusiast Robert Duquemin who discovered a silver Anglo-Saxon clothing fastener in a field near Barbury Castle, thought it was a piece of rubbish.
Police discovered the item at Mr Duquemin's home in Home Farm, Highworth.
He was charged with several offences, including using a metal detector without consent and damaging an ancient monument.
However, these charges were dropped in April last year.
Mr Duquemin told police he didn't register the find, as required by law, because he had no idea of its historical importance.
Wiltshire coroner David Masters declared the item was officially treasure at an inquest on Friday, and it will be now be displayed in a museum either in Swindon or Devizes.
Under the Treasure Act 1996, items over 300 years old and containing at least ten per cent precious metal can be seized by the Crown.
Katie Hinds, finds liaison officer for Wiltshire, said the triangular hook, which dates from between the 9th and 11th centuries, would have been used to attach veils or other garments. It's value it not yet clear.
Experts from the British Museum said the hooks were fairly common during the early medieval period.
They have been discovered in Southampton, Winchester and Yorkshire.
Mr Duquemin said couldn't be sure of the date he found the hook, but thought it was probably in September 1998.
Mr Masters told him he was required to register anything that may be treasure with 14 days.
Mr Duquemin said he was unaware of its value. In police interview he said: "I thought it was a piece of rubbish."
Mr Masters pointed out that he also told police he thought it could be a Tudor hook.
"You have been a metal detector for 15 years and you thought it looked like a piece of rubbish," he added.
"I do realise now because of what I've been told," said Mr Duquemin.
"But I'm not an archaeologist, and I've never come across anything like it before."
Mr Masters said he found it hard to accept this argument.
He said it was a valuable part of Wiltshire's heritage.
"It is a finely manufactured piece of silver in a nearly pristine condition," he said.
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