Mme. Loisel does not recognize good quality jewelry. She does not for a moment recognize or suspect that the diamond necklaces she borrows from her wealthy friend is a fake, a mere piece of glass. When her friend, Mme. Forestier, tells her to look around for a necklace to borrow:
Suddenly she discovered, in a black satin case, a superb diamond necklace; her heart began to beat covetously. Her hands trembled as she lifted it. She fastened it round her neck, upon her high dress, and remained in ecstasy at sight of herself.
Mme. Loisel is too poor to have developed an eye for jewelry, but the point of the story is that she is too shallow, too fixated on longing for luxury, to probe beneath the appearance of things. The fake, paste necklace, which glitters but has no real value, represents the shallowness of the life Mme. Loisel covets. Mme. Loisel also never questions her friend about the value of the necklace after she loses it, which represents the unquestioning way she accepts the outward form of the wealthy life as having great value. If she had looked beneath the surface at all, she would have recognized that what she desired, the necklace and the life, were worth far less than she imagined.