The day after Madame Loisel loses what she thinks is a highly valuable necklace, she goes to the jeweler whose name is inside the necklace's empty case. She hopes that the jeweler can help her find an exact replacement for the missing necklace before Madame Forestier finds out. Unfortunately, the jeweler is not much help. He closely consults his sales records and finds that he hasn't recently sold any such item matching the necklace's description; he must have only supplied the case. This is ominous news indeed, but Madame Loisel is either too slow to draw the necessary conclusions or perhaps just so desperate to track down a replacement that she doesn't catch on.
At any rate, the episode at the jeweler's foreshadows Madame Loisel's subsequent discovery, many years later, that the lost necklace wasn't really a valuable piece of jewelry at all, but a fake. If it really had been such a precious item it would have been supplied along with the case.