Wow. This is a knock out.  Lovely Pearl with tested diamond on original Signed 18k 20”+ chain.  The Pearl is approximately 5.9 mm according to my little chart. I Believe this is a Gucci link chain.  It is beautiful. I am having trouble parting with this but my son gave me a similar Pearl necklace so I will wear that one.  Enjoy this beauty.  I’ve owned about 15 years. Excellent condition. Chain is signed 763VI and an outlined letter M a star and an twirly flower or sun emblem. And on the other side 18k Italy. Another ebayer has just the similar chain for $2,399.99.