Maupassant does not describe the borrowed necklace in detail. He simply writes:
Suddenly she discovered, in a black satin box, a superb diamond necklace, and her heart throbbed with an immoderate desire.
Later when the Loisels are trying to find a duplicate of the necklace Mathilde has lost, they find one at a jewelery shop which is only described as follows:
a string of diamonds that seemed to them exactly like the one they had lost. It was worth forty thousand francs.
No doubt Maupassant did not want to spend many words on the design of the lost necklace because he was deliberately misleading his readers by making them think the diamonds were real. As the author of the story, Maupassant would of course know the diamonds were "paste," or fake, even though his character Mathilde Loisel did not. Her friend Mme. Forestier also knew the diamonds were not real, as she reveals to Mathilde at the end of the story. The black satin box makes the necklace seem more valuable. Apparently the necklace is a simple "string" of jewels that encircle the neck and not one of those complex clusters that hang down across a woman's bare upper chest. The fact that the necklace was a fairly simple "string" would make it easier to find a duplicate, whereas it might be hopeless to try to find a duplicate for a more elaborate creation.