Thank you for contacting Mearto with your appraisal inquiry.

It is a beautiful necklace. Such vintage jewellery are being offered at auction on a regular basis.
The design and choice of precious stones for the necklace is very elegant and beautiful. However it is hard to determine the quality, colour and number of inclusions of the larger diamond based on a photo alone. The necklace itself is according to your description made of 18 kt gold.
The estimate provided is for a well-advertised specialised online jewellery auction, where I believe the necklace has the best chance of realising its fullest potential. However, there is no measurement for the sentimental value you might attach to the lovely necklace. Retail "asking prices" can be higher and vary.

Please let me know if you have any additional questions. I’m happy to help.
Thank you