This is the first time we have encountered this particular ‘gemstone’. Operculum is essentially a retractable trap door that gastropods engage when retracted. In short, as far as we can make out from wikipedia, it is the door to a snails home. I would not call it rare but it is most definitely a slightly obscure thing of which we have never had in stock.
Metal: 9ct yellow gold.
Marks: PP Ld, 9, 375, 1935.
Measurements: 18mm x 16mm on the table of the ring and 8mm proud of the finger with shoulders as wide as 7.5mm and then narrowing to 4mm at the back of the finger.
Stones: Operculum.
Weight: 7 grams.
Condition: Excellent.
Finger size: UK P, USA & Canada 7 3/4.