This is a highly unusual antique Georgian or at least early Victorian period sterling silver brooch set with a center stone of translucent moss agate surrounded by 12 iridescent sea shells. The shells are set in closed back settings. The agate has an open back. There are no markings but the metal tests positive for silver content of sterling grade. The back has an old fashioned T hinged pin stem or exceptional width, and a C clasp closure. There is also a loop with a short length of chain attached that would have once been a safety chain and additional pin, but a portion of that length of chain and the safety pin are gone. However that loop makes it possible to add a bail and wear this brooch as a pendant if desired.
This brooch measures 2.125" x 2". The center stone measures 25mm x 30mm. The shells are each about 8mm in diameter.
Condition is overall good. I noted the missing safety chain and safety pin. Also the small silver half domes between the shells along the outer edge are pushed inward along that outer edge. I'm not sure if that is a defect or the way it was made.
I have been dealing in antique jewelry a long time and this is the first time I have come across this combination of a stone with shells. Very unique.