Enotes has an excellent response to your question at the URL below. The necklace itself represents the theme of appearances versus reality. “While sufficiently beautiful to make Madame Loisel feel comfortable during the ministerial ball, the necklace is actually nothing more than paste and gilt. Thus, it is not the reality of wealth or high social class that is important for Madame Loisel, just the appearance of it.” As the discussion on enotes points out, other themes involve conflict of class (rich vs poor) and greed and generosity. The irony in the story is that Madame Loisel ruins her life in trying to compensate for the lost necklace, when in fact the necklace was not worth anything to begin with. As for symbolism, it resides in the title which also carries the theme: the necklace symbolizes the difference between appearance of and reality, a wealth that is empty, having no real value.