The diction of a story or novel refers to the use of language that the author chooses for the narrative as well as for the characters. When the story is complex and controversial the language use might be more researched, emotional, and perhaps even adventurous.
A story like "The Necklace" is told from the focal point of view of a woman of a lower social station, who aspires for bigger and better things to a point of exaggeration. This could be close-read as a parody of a neurotic and hard-to-satisfy woman that gets what she deserves at the end. However, the story could also be read from the tragic perspective of a woman whose origin and circumstances render her unable to aspire to better or beautiful things.
The ambivalent way in which the story can be interpreted calls for language that would fit either perspective. This is why Maupassant's choice of language is simple, dry, and to the point.
The girl was one of those pretty and charming young creatures who sometimes are born.. into a family of clerks. She had no dowry, no expectations, no way of being known, understood, loved, married by any rich and distinguished man; so she let herself be married to a little clerk of the Ministry of Public Instruction.
There is no figurative language that would allow the use metaphor, or personification, for example. There is a strong element of naturalism in the narrative which means that the story telling lacks sentimentality.
In contrast to Mdme. Loisel's fantasy world, her husband appears to be a simpleton; he is happy with his soup, he is glad to be invited to the Ministry's Ball, and he is content to take a minor participation in it. His language is equally simple and quite fitting with his characterization which is almost entirely indirect.
The end of the story is quite characteristic of French short story in that they end almost abruptly, citing perhaps a quick afterthought, or a sudden reaction from a character.
You say that you bought a necklace of diamonds to replace mine?"
"Yes. You never noticed it, then! They were very similar."
"Oh, my poor Mathilde! Why, my necklace was paste! It was worth at most only five hundred francs!"
This abrupt, yet, simple ending is part of the diction; nothing is surprising except the content of the dialogue and not the form in which it is presented figuratively nor aesthetically. Therefore, three ways to describe diction in "The Necklace" is naturalistic, simplistic, and unemotional.