Buried in a museum, in this case, but treasure just the same. A lump of “organic material” from a 19th-century archeological dig in Norway had been kicking around the storerooms of the British Museum for over a century before one of the curators noticed a bit of metal sticking out and decided to have the whole thing x-rayed.
His hunch paid off. Inside the lump of material was a Celtic brooch dating from the eighth or ninth century — six centimeters in diameter, gilded, and decorated with interlaced knotwork patterns.
The whole story, with a picture of the brooch in question, can be found here.
(Obligatory writing reference: Sometimes it pays to look more closely at the weird lumps of old or unpromising story-stuff that are kicking around your head. There may be something valuable hidden inside.)