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.