The reader never really finds out where, exactly Madame Loisel loses the necklace. But, the author suggests that when she is getting ready to leave the ball, and it is very late, and she has had a great time, and does not want the feeling of joy and excitement that she has experienced by tarnished by the thought of her actual life at home, that it causes her to resists the efforts of her husband who is trying to put her ragged coat on her shoulders as they venture outside to get a cab home.
It is at this point, when she is leaving the ball, and looking for a cab, or perhaps in the long walk that they make to find a cab or in the old cab that they take home that she loses the necklace. She does not discover the necklace gone until she is at home and getting undressed.
As a result of the ten years of hard physical labor that Madame Loisel and her husband perform in order to pay off the debts incurred to buy a replacement necklace, she has lost her former beauty. Her hair has grown coarse, her hands are red, dry and swollen from scrubbing and cleaning, she is at the end of the story, the picture of a poor scrubber woman, everything that she despises at the beginning of the story. Gone is her beauty, which she believed put her in a different social class, far from the coarse, simple lower class girls.
"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."
At the end of the ten years, Madame Loisel looks far older than her age would indicate. So much so, that Madame Forestier does not even recognize her when she encounters her in the park.
Trading her life for a brief moment of acceptance into the world of the rich and priveleged, Madame Loisel experiences one night of joy and many years of suffering and sacrifice.