The item was a signet ring, one of several items found in a pouch on the body of a murdered man (check out what Lucy is holding on the cover). This ring, along with a hodgpodge of other eclectic items, will help identify the victim, as well as his killer.
The signet ring I describe in the book was unusual because it swiveled to allow the wearer to display one of two different images.
I had come across this interesting ring style when I was prowling about on the British Museum's website. The ring featured here is from the early 17th century, and was made in either France or Germany.
When you really think about it, what would be the reason to have a ring with two faces? Boredom? Perhaps. Cost? Unlikely, since a swivel ring might be very expensive to create. Or perhaps, there is something about who you are that you wish at times to keep private, and other times, make public. It is not surprising that certain organizations, like the Freemasons, have used such rings since the early modern era.
This particular ring was probably more ornamental than practical, but I liked the idea that there could be a secret hidden beneath its surface.
This ring is a little more elaborate than I was envisioning though. Not, perhaps, as simple as these more simple masonic rings (a style also found in the 17th century), but somewhere in between.