When Goldie was forced to work for one month in Scrooge 's claim in the White Agony Creek , a bear attacked her and grabbed her by the hair. In order to save her, Scrooge threw a bowie knife, tearing apart the "golden" lock that the bear held.
He saved the lock in his safety deposit box , and would tend to watch it every night before sleep. When Goldie stole the deposit box a few days later, she realized Scrooge's love to her, and decided to return to the White Agony Creek .
Scrooge continued to keep the lock in his deposit box, even after leaving the Yukon . When his sisters, Hortense and Matilda discovered it, he was clearly embarassed but didn't tell them whom the lock belonged to.
It is implied both during his Klondike years, but also his final years that he considered it the most valuable item he ever had, as it symbolizes the best time of his life. Scrooge's actual most "valuable possesion" is in all likelihood his memories.