Ring around the Rosary
The idea of a ring-shaped device with ten "bumps" for the ten Hail Marys seems like the sort of thing that could have been invented any time. But what attracted my attention to the history of the idea was someone selling replicas on eBay a couple of years ago of a supposed "16th-century" English rosary ring. They were only asking a couple of dollars, so I bought one and added it to my teaching collection.
I didn't find the story the seller was telling very convincing -- he claimed it was "found in a field where in the 1500’s a priest was said to have been hung". Someone who knows a little about the persecution of Roman Catholics in England would begin by pointing out that (1) it wasn't illegal simply to be a Catholic priest in 1500s England, in fact there were plenty of them left over from the old days, still living quietly on their pensions, that (2) active persecution of missionary priests didn't start till well into the 1580s, that (3) when missionary priests were found they were treated as State prisoners rather than simply hung, and (4) anyway that merely finding something in the same field where such a thing happened doesn't mean there is any connection. However.
This still left me looking for historical evidence.
At this point the only actual rosary rings I'd seen were modern. Various styles of modern rosary rings show up pretty regularly on eBay and in religious goods stores. Most of the modern ones look like these:
I imagine they'd be quite uncomfortable to actually wear on a finger, but they're really designed more as something to carry in one's pocket for easy (and perhaps discreet) use. They have an advantage over strung beads in that beads can't possibly come loose or fall off. Sometimes they're referred to as "soldier's rosaries."
Another modern version looks rather more comfortable for actual wearing. I've seen several versions of these as well:
Much to my surprise, however -- considering my skepticism about the supposed 16th-century replicas on eBay -- when I finally had the time to start combing online museum images for period rosaries, the Victoria & Albert Museum turns out to have one, which they date to 1500-1520. Here's a small version of their picture.
Unfortunately the way the V&A online image collection is set up makes it hard to create direct links, but if you go to http://images.vam.ac.uk/ and enter 775-1871 (the object number) into the Search box, that will take you to the page featuring this ring, where they have some interesting things to say.