The Headpiece to the Staff of Ra
A simple bronze medallion with a crystal in the middle and some writing on the side. The key to the most fabulous artifact in the history of man. Karma in action for evil Gestapo agents who get a little too grabby with it. The Headpiece is all these things and more: an artifact from the city of Tanis that tells those who carry it exactly where to find the Ark of the Covenant.
On a basic level, the Headpiece acts a little bit like a mini-Ark: inflicting righteous pain on the unworthy (*cough* Toht *cough*) while guiding Indy and his friends where they need to go. On a slightly deeper level, it's a symbol of the past: in this case, Indy and Marion's past as much as the past of ancient Egypt. Abner Ravenwood gave it to Marion as a present, so it has connections to him and, by extension, to the pain that Indy inflicted on his daughter.
But it also represents Abner's continuing hold over Marion's life. Seriously, she doesn't care about Tanis. "Abner was sorry for dragging me all over this earth looking for his little bits of junk," she growls about her father, which doesn't exactly sound like a happy childhood. Her letting it go means that she's willing to rethink her relationship with Jones and, more importantly, that their mentor/guide no longer has influence in their lives. They're ready to go it alone.
That's a lot for one little medallion, but it makes for a neat way of conveying some important themes in the middle of all that Nazi-hand-burning.