The story's description of the watch chain is an example of visual imagery. Della has spent months saving and planning to buy Jim "something fine and rare," "worthy of [...] being owned by" him. After searching many stores, she finds the perfect thing—a unique platinum fob to go with his grand pocket watch, a family heirloom:
It was a platinum fob chain simple and chaste in design, properly proclaiming its value by substance alone and not by meretricious ornamentation [...] It was like him. Quietness and value—the description applied to both.
The imagery of the watch chain reflects Della's high opinion of Jim, as well as the fact that both Della and Jim value their marriage's substance over external style. Fob chains were fashionable in the early 1900s, when the story is set. The pocket watch would be attached to one end of the chain and kept in a pocket, and the chain would be passed through buttonholes and anchored in an opposite pocket.
"Quietness and value" apply to both him and the watch chain. Like Jim, the watch chain isn't showy and doesn't draw attention to itself, but it has inherent value that's clear to someone who takes the time to look. Its quality consists in the fact that it's well made and useful. The word "meretricious" can mean tawdry or pretentious and can even be associated with prostitution; the watch chain, being "simple and chaste," lacks these flamboyant qualities. Jim, similarly, is thin, serious, and unassumingly dressed ("He needed a new overcoat and he was without gloves"), yet in spite of his obvious lack of wealth and his unimpressive exterior, he is a kind, steady, substantial presence. Like the watch chain, Jim must be studied closely so that his inner value can be appreciated.
The image of the "simple and chaste" watch also shows what Della values most: Jim's internal qualities. Though she buys the fob chain because she believes it expresses those cherished qualities and she wants to honor those same traits ("quietness and value") in her husband, she clearly values intangible character much more than material wealth. And so does Jim—when he later appears in the story, his own actions make it clear that although he's very proud of his heirloom watch, he values his wife more. Selling the watch means a family heirloom is gone forever and that he can't even use the watch chain Della has gifted him, but it also suggests that expressing love to his wife is more important to him than even the modest status symbol of a handsome accessory.
Della's long brown hair is one of the Youngs' only prized possessions, and cutting and selling it becomes Della's only hope of affording a Christmas present for Jim. The story uses visual imagery to convey the hair's uniqueness and value:
Had the queen of Sheba lived in the flat across the airshaft, Della would have let her hair hang out the window some day to dry just to depreciate Her Majesty’s jewels and gifts.
The image of Della's hair as so precious and beautiful that it would actually cause the value of the Queen of Sheba's treasures to decrease is obviously nonsense, but it does reflect how valuable Della's hair is to her. She has grown and tended her hair for years, perhaps her whole life, in order to reach this length. So Della's hair isn't a matter of simple vanity, but nostalgia, too—it's been with her for years, rather like Jim's beloved pocket watch. The value of her hair also underscores the fact that the Youngs simply don't have many material possessions, if hair is one of the most valuable things they can boast.
Just after this, Della studies her reflection in the mirror, weighing her options, her hair falls below her knee:
So now Della’s beautiful hair fell about her rippling and shining like a cascade of brown waters.
Instead of simply saying that Della's hair is long, the story uses rich imagery to liken it to a waterfall whose waters ripple and shine. The comparison imbues Della's hair with vitality—it's not just a lifeless, inert thing, but a shiny, flowing substance, practically a natural wonder that pulls people's gaze to it. This liveliness also reflects Della's own bright, lively personality. The imagery further reinforces the value of Della's hair and, in turn, how much she loves Jim, that she's willing to get rid of something that's so closely connected to who she is.
This imagery is repeated later in the story, when Della's tearful farewell to her hair contrasts with Madame Sofronie's brisk appraisal of it:
"Take yer hat off and let's have a sight of the looks of it." Down rippled the brown cascade. "Twenty dollars," said Madame, lifting the mass with a practised hand.
Again, describing Della's hair as a "brown cascade" that "ripple[s]" emphasizes its beauty, brightness, and vitality. Selling her hair is a life-changing sacrifice for Della, but for Madame, it's just another business transaction. When she matter-of-factly weighs Della's hair—"lifting the mass with a practiced hand"—Madame acts like a merchant who assesses any other type of goods on a daily basis. To her, there's nothing unique about this head of hair, and it's interchangeable with any other specimen in her eyes. She doesn't even notice that Della's hair is a "brown cascade," a natural wonder that reflects Della's personality (and is irreplaceable in that sense). In light of the earlier imagery that conveyed the hair's personal value, Della's hair is clearly worth much more to her than $20, but in her eagerness to use the money to buy a gift for Jim, she doesn't mind the paltry sum.
All in all, the visual imagery used to describe Della's hair emphasizes how much Della treasures it. In turn, it's clear that selling it is a great sacrifice, and that she treasures Jim much more, wanting to be able to buy the perfect gift for him. In the end, it's that readiness to sacrifice her cherished hair, not the pricey watch chain the hair pays for, that most conveys Della's love for Jim.