What symbolic meaning does the necklace have for Madame Loisel?

The necklace represents affluence.... it is a sign of status and wealth.

She replied: "I'm upset that I have no jewels, not a single stone to wear. I will look cheap. I would almost rather not go to the party."

Suddenly she discovered, in a black satin box, a superb diamond necklace, and her heart began to beat with uncontrolled desire. Her hands trembled as she took it. She fastened it around her neck, over her high-necked dress, and stood lost in ecstasy as she looked at herself.