From the Christie's Web site.
Catherine is portrayed wearing strongly arced crescent necklines, including this one.
Keywords: Clouet, Catherine de Medici, Medici family, Valois family, Queen, French, curly coiffure, hair jewelry, jeweled headdress, split partlet, high enclosing neckline, ruff, bodice, bodice, crescent neckline, full sleeves, puffed cloth ornaments, carcanet necklace, draped necklace