Thank you for contacting Mearto with your appraisal inquiry.

It is a beautiful little brooch. Such antique jewellery are being offered at auction on a regular basis.
The design of the brooch with the black background/onyx set with stones to resemble a flower is very charming. I am not sure however that it is diamonds, it could also be rock crystals which were popular in the 1800s. I would date the brooch to be from the middle to late 1800s. So I quite agree with you on the dating of the piece. Despite the age it is not the type of jewellery that fetches the highest prices today as fashion today prescribes diamonds, however I still believe that the brooch would attract the attention of both collectors and dealers if it came up for auction.
The estimate provided is for a well-advertised specialised online antique jewellery auction, where I believe the brooch has the best chance of realising its fullest potential. However, there is no measurement for the sentimental value you might attach to the brooch. 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