This is a traditional form of necklace worn -- mostly by women and girls -- both as an expression of religious faith and as something that is thought to provide protection and luck. One common form is a string of beads more or less in the pattern of a rosary, with marker beads at intervals and with a large number of medals, symbols, "relicarios" and "milagros" on it or hanging from it.
A "relicario" is not necessarily an actual saint's relic, but a small religious painting, print or carving set in a metal frame. There is a long tradition of making and wearing relicarios as personal ornaments, dating all the way back to the Spanish conquest. I happened across a very interesting book on the subject, Relicarios: Devotional Miniatures from the Americas by Martha J. Egan, which has abundant pictures and good discussion.
"Milagros" are small cast-metal ornaments similar to the votive "offerings" often given to shrines: little images of the body part or other thing that's felt to be in need of protection or healing. Like the relicarios, these are still being made and sold today. I bought some through eBay that seem to be cast in tin or pewter -- they're very lightweight and very roughly finished. I've seen milagros in the shapes of eyes, hearts, arms and legs, cows, chickens, and male and female heads or praying figures that may represent children or other relatives, or perhaps saints. I was pleasantly surprised to see that Martha Egan has a book on these as well: Milagros: Votive Offerings from the Americas.
Here's a closeup of the traditional milagro necklace from the Relicarios book shown above:
This has groups of ten beads between the marker beads or gauds, so it could be used as a rosary, but it has twelve decades (groups of ten) rather than the five or fifteen decades one would expect from a rosary.
Here's one that came up for sale on eBay a while back. This one's much less regular in its construction.
I think the question of whether these are really "rosaries" or not has to remain open. Certainly they're often described that way by people who may be a bit hazy on the concept of just what rosary beads should look like. But the final definition of a rosary is whether it's used for rosary prayers, and of course there's no way we can determine that just by looking at the beads.
By the way, if anyone's interested in either relicarios or milagros, Googling on either one seems to turn up plenty of links, including sellers of both antique and new examples.