What was Jim's most prized possession in "The Gift of the Magi" by O. Henry?

In O. Henry's short story "The Gifts of the Magi," James "Jim" Dillingham Young's most prized possession is the gold watch that belonged to both his father and his grandfather and has been passed down from one generation to the next. This watch is so fantastic that O. Henry claims,

Had King Solomon been the janitor, with all his treasure piled up in the basement, Jim would have pulled out his watch every time he passed, just to see him pluck at his beard from envy.

Unfortunately, the Dillingham Youngs are not a wealthy couple. With Christmas approaching, Jim decides to make a huge sacrifice: he pawns off his watch in order to get money to purchase a lovely set of tortoiseshell combs for his wife, Della, whose beautiful hair is her most prized possession.

When Jim arrives home that night, he discovers Della cut off and sold her hair in order to purchase a chain for his watch. The pair is stunned by the sacrifices they each made that also ultimately render both gifts useless. Despite this, they are incredibly moved by their love for one another, and this willingness to give from the heart with no concern for one's own happiness serves as the lesson of "The Gift of the Magi."