Mansfield leaves some things ambiguous, so the story can be interpreted in different ways. But here is one analysis of the symbols.
Bertha decorates the table and the fruit on the tray to look as lavish as possible. The tray includes "white grapes covered with a silver bloom"—the entire display seems to float (to Bertha) and in her state of bliss, she laughs at the sight. When Pearl arrives, she is described as "all in silver, with a silver fillet binding her pale blond hair." When Bertha and Pearl look upon the garden, they look at the flowering pear tree seeming to reach "the rim of the round, silver moon." There is silver associated with the moon, which traditionally has been associated with femininity. The tree, sometimes understood as a phallic symbol, reaches toward the moon, clearly a sexual symbol which will be discussed below. Pearl's silver is also about femininity, beauty, and somewhat for show. The silver "bloom" of the grapes represents "fruitfulness," life, but also showiness. In short, silver symbolizes a few things here: a kind of static, frozen showiness; beauty and femininity; and also a connection Bertha feels with (or for) Pearl.
The "shower of sparks" describes Bertha's feeling of bliss. This is a rush of life filling her mind and body. There is no reason given for why Bertha feels this bliss. After reading the story, one might conclude that she simply has moments of appreciation for a comfortable life. On the other hand, given the subtle critique of this bourgeois lifestyle, the bliss/shower of sparks is a delusion, something Bertha contrives in her mind to convince herself that her life is indeed wonderful, rather than simply a series of superficial silver, glowing objects and inane party guests.
The pear tree might be the most important symbol in the book. Bertha has (or convinces herself that she has) a dream of a life. When she looks at the pear tree in its perfection, she connects with it, sees it as an extension of herself. Bertha claims (to herself) that she is quite happy and leads a near perfect life. The pear tree "stood perfect" with not a single "faded petal." Bertha is the tree. She sees Pearl as something mysterious and perfect. In fact, there are interpretations of this story that say Bertha is sexually attracted to Pearl. (What supports this idea is the statement late in the story when Bertha feels attracted to her husband for the first time. This could indicate she is/has always been homosexual or bisexual. Therefore, her attraction to Pearl is understood.) And it is probably no mistake that the name "Pearl" and the "pear" tree are identical except for the final letter "l" in Pearl's name. If Bertha is the tree and the silver connects Pearl to the moon, then Bertha as the tree reaches toward the moon (Pearl). The symbols of silver and the tree combine with Pearl, all with notions of femininity, beauty, sexual attraction, and in the end a silver symbol of being on the surface.
This last interpretation of superficiality arises from the final disillusionment of Bertha's wonderful world. Her perfect world, frozen in silver, also symbolized by the tree, is shaken when she sees Pearl being intimate with Harry. Bertha had idealized her life to the point of bliss; this silver, shining life is her ideal vision. The reality of it is shaken in the end but Bertha could always just live in an ignorant delusion. The final line indicates that this delusion of the great life remains: "But the pear tree was as lovely as ever and as full of flower and as still." One conclusion is that Bertha will continue to live in her ignorant delusion because ignorance is bliss.