What symbolic meaning does the necklace have for Madame Loisel?

From the very beginning, Loisel feels inadequate. She longs to go to this party, yet she doesn't "have a thing to wear." She and her husband happen to live in the "lower bourgeoisie, a class that stands above tradesmen and laborers." So she has some money, does better than her parents did, but she longs to live the life of luxury.  Her main conflict is an inner one.  She wants to be a part of the upper class, but she will never attain that life. In fact, her actions end up sending her further down the ladder of social standing.

This necklace, which she thinks is genuine, represents that lifestyle.  It symbolizes everything she longs for in her materialistic life.  When she loses it, she is so embarrassed that she lies about it, purchases a real necklace in its place, and then suffers the rest of her life making up for its loss. From the beginning, she felt that she deserved it.  However, as she worked for the next ten years of her life, she realized her own true worth.  The irony is that the necklace itself was just as worthless as her past ten years had been.