Thank you for contacting Mearto with your appraisal inquiry.

It is a beautiful pendant. Such vintage jewellery are being offered at auction on a regular basis.
It appears to be made of silver, and judging from the design the pendant was made in the early 1900s where this type of jewellery was modern. The stone is hard to determine from the photos. Green agate was a popular stone in this particular era, however it could also be malachite. Either way the estimate will be the same.
I am sure that the 2 bumps on the back of the pendant are for attaching the stone as you describe.
I cannot determine the material behind the green stone - It may be oxidised silver or perhaps black onyx.

The estimate provided is for a well-advertised specialised online jewellery auction, where I believe the pendant has the best chance of realising its fullest potential. However, there is no measurement for the sentimental value you might attach to the sweet pendant with the green stone. 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