“Enormous” carbuncle garnet, hollowed out in the back to let more light through. These size garnets were very rare in the Middle ages / so much so that some of them made it into Royal crowns../ and were highly collected and prized.
This one / might have come from an earlier “Kunst Kammer ” / is 30 x 27 mm and the height is 15 mm
The setting is French ca 1870-80 gold and silver , set with diamonds and a natural black pearl
Measurement in vertical : 60 mm