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They sell them to maintain a medallion that is important or a spiritual or a crucifix or a necklace as a charm, or as a keepsake, with a value. For those, it's for the women mentioned previously and for beauty itself.
Historical figures had done giving a necklace. In 1811, a diamond necklace was introduced by Napoleon Bonaparte as a present, for his wife, Marie-Louise, Empress of France. The necklace includes bits of diamonds and contains 263 carats' burden.
Giving necklace as a gift could be viewed also in literature, such as the story “The Necklace" composed by the famous French author Maupassant. The giving of a necklace might be romantic, as much as giving a ring might be symbolic.