It’s easy to see the jewels in the story as symbolic of a betrayal of trust. The wife, who had been the mistress of a wealthy man for some time, clearly had been too subtle for her husband; her “fake” jewels, which he only realizes after her death are worth thousands, are evidence of both her duplicity and his gullibility. No wonder it is such a shock for him!
Maupassant might be trying to get at something else, though, because it is also true that the man was happiest when he was ignorant of his wife’s affair. Considered this way, the jewels might represent the ambivalent nature of truth. For example, it is ironic that the “fake” jewels – jewels that pretend to be real – are in fact real jewels pretending to be fakes. Arguably he would have been better off not knowing the “truth” of the jewels or of his wife.
It’s also worth mentioning that there is not much evidence in the story that his wife was unhappy with her husband, either, adding a further ambiguity. It’s entirely possible that she was happiest when she was able to pretend with her husband that the jewels were fake, and that she had been faithful. In this way, the ambiguous nature of the jewels might represent the wife’s own divided feelings.
This move also goes double for Maupassant’s narrative strategy. Maupassant’s narrator is not exactly reliable, since he too is participating in a deception – he does not reveal the truth about the jewelry until the end of the story, so in effect the reader experiences the same twist and disappointment as the man. Considered this way, the jewels might represent unreliable nature of narration itself.