not enough pictures, I can't see if it is a genuine Art Nouveau brooch or a reproduction and although the master mark seems to be French, I need to see the eagle's head mark to prove that. Elkan Wijnberg,
http://www.adin.be
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French Art Nouveau Designers by: Art Nouveau Jewelry Fan
Two famous French Art Nouveau Jewelry Designers with the initial "G" were Lucien Gautrait and Lucien Gaillard. However, I believe they signed their work, L. Gautrait and L. Gaillard.
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More Information Requested by: Help Elf
Please send through any background information about the art novueau pin you might gave. Where did you acquire the piece? If you could also send through a 2nd photo of the hallmark itself, the back of the pin. Also the dimensions.