An old thread . . . but my first reply to it. I think the Holy Grail would be something of great historical significance, and I've got a pretty interesting story along those lines:
We lived in a Victorian house until six years ago, and our next-door neighbor was a ninety-plus-year-old woman who lived in a pre-Civil War home built by her grandfather, Levi Shafer, an member of the 8th Illinois Cavalry. According to many historical documents, Lieutenant Marcellus Jones borrowed Shafer's Sharps carbine to fire the shot that commenced the Battle of Gettysburg:
http://www.historynet.com/battle-of-gettysburg-who-really-fired-the-first-shot.htm.
Like many things in history, when and who fired the first shot at Gettysburg is disputed, but this account is nonetheless accepted by many as historically accurate. At any rate, this carbine played a pivotal role in the Battle of Gettysburg, and, at a minimum, it would be rare in the sense that there was a very specific, historically significant story attached to this specific firearm.
Anyway, according to my neighbor, Jane, this carbine was in the basement of her home for some period of time. In fact, I saw other artifacts from Shafer's service--parts of his uniform, his discharge certificate, etc.--but at some point the carbine disappeared. Whether some family member or visitor grabbed it and sold it, or a neighborhood kid liberated it, she did not know. It was one of those things buried in a basement, and it simply "disappeared."
Jane died a couple of years ago, and her family sorted through all the possessions in her house. They sort of halfway hoped that they'd locate the carbine buried among the ancient stuff in her basement, but it was nowhere to be found. Unfortunately, it's probably lost for all time. However, something like that would be incredibly cool, having played a pivotal role in American history.