This beautiful necklace is part of the Signature X Collection. This collection has earrings, cufflinks, bracelet, and necklace. It is a classic Tiffany design. The necklace is clearly stamped "750" and "Tiffany & Co." It would have been made for Tiffany & Co. in Europe, probably Italy. The necklace is the heavier version of this design, weighing 110 grams. It measures just over 16 1/2 inches and is in excellent condition. It is finished by a tongue in groove clasp and a buried safety catch.
At this point, the companion bracelet "Signature X" for Tiffany & Co. is also part of the collection and you can see it online as well as this beautiful necklace.