Could it be a button?

[shiningstar76]

a button possibly? backside is hollow, piece is light, about the size of a half dollar.

 

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[SparklyStuff]

Nouveau style brass pendant/medallion

http://www.tias.com/stores/glitzqueen/

look under Necklaces & Pendants - Victorian & Edwardian

There are some examples of these beauties and the findings that put them together into necklaces on that site.