Identify Your Jewelry: Black Cameo with Oval Bezel - Appraisal & History

Question
Hello...I just found this piece of jewelry and was wondering if you could help me identify some history...it is a black cameo of a lady looking to her left..it looks like the cameo is mounted on a black foundation, or it could be one piece I suppose.....the entire cameo is set in a oval bezel with a reticulated scalloped border.  The back of bezel is tooled in a swirl pattern and has the words pr st cc/1/20 12k over Sterling. There is an incredible crafted hinge that allows for the back to open to reveal the back of cameo..what is the purpose of it opening..also there is no loop to run a chain thru to wear as a pendant.....no sign of it ever being a ring either.  The cameo is approx. 1/4" thick...any help would be greatly appreciated.....thanks so much

Answer
Hi Jack,
Although this is not my area of expertise, we have had made many opal cameos in germany over the years . This uses opal on top  and a backing of either onyx or agate for a backing. Cant help with the pr st cc etc / perhaps the markers mark .Its very difficult to access the item without seeing it.The work sounds quiet elaborate and my guess is that it may even be of italian origin . sorry its not much. best regards Rod