Let's look at the symbols in "Gift of the Magi" for a moment:
1. Jim's watch: symbolic of the concept of time, such as the time that has passed from his receipt of the heirloom watch until the present in the story. It is also tied to the progression of time, such as that required for one's hair to grow (see #2). The watch also represents quality, as it is gold.
2. Della's hair: Like Jim's watch, the hair represents the passage of time, or the attainment of it, as it has taken a great deal of time for Della to grow out the long locks she possesses. When the hair is cut and the watch is sold, those acts represent the sacrifice of time and effort in the name of love. The irony that exists is that each person gave up their most precious symbol in order to supplement the other person's, which was also given up. O. Henry was a master of bittersweet irony.