This is the last remaining gold laurel leaf from the wreath that Napoleon wore at his coronation in 1804. The leaves (and berries) - all fashioned by the court jeweler Martin-Guillaume Biennais in naturalistic style - were later melted down by Louis XVIII during the Restoration. This leaf survived because Napoleon gave it to the painter Jean-Baptiste Isabey, who later had it mounted in this box with an accompanying note about how he came to have it:
"At Saint-Cloud in 1805, before the departure for Milan, I was helping the emperor as he tried on the royal crown which was supposed to go above the golden laurel wreath made for the coronation in Notre-Dame. One of the leaves fell off. Just as I was about to give it to the head chamberlain, His Majesty said to me: 'Keep it; it will make a good souvenir of your clumsiness.'"
It is worth listening to the short audioguide for this piece.
Quotation via Napoleon.org