This appears to be a Victorian mourning brooch set with a faceted black stone (onyx? glass? jet?) surrounded by a fine gold mesh.  It is enclosed with a twisted silver border.  The stone is bezel set in a gold rope border.  The closure appears to be a trombone type that is missing its cap.  It is still very functional and secure.  Please look at all pictures closely for condition as it is quite old.  I've not seen another piece like this and it is truly unique. 

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