In the story “The Necklace”, the lost necklace stands for lost hopes and dreams for Madame Loisel.  It also represents her greediness and her disingenuousness because she only judges herself by what she has, and it is her belief that others will do the same. The necklace with the  artificial diamonds represents the artificiality of her character. People admiring the necklace only for its supposed value have been deceived. Just because something looks real doesn’t mean that it is. Just because Madame Loisel looks upper-class at the ball doesn’t mean that she is.   The real necklace stands for honesty and originality, which are not represented by Madame Loisel at all, but do seem to come across in her poor beleaguered husband and her friend.

The meaning of the lost and real necklace are intertwined. When the Loisels first borrow the necklace, they think it is real, and, while they know they don't own it, they think that if it looks like she/they own it, their own value will be increased. Appearance will trump reality. Once they lose it, they have to buy a real replacement. They must work hard to pay for it, and in the process, Madame Loisel loses much of her original beauty, which was real.

The meaning of both is that if you confuse appearance with reality, you will suffer, and that making an illusion real (paying for a real replacement) would cost intense work.
Greg