First of all, let me say that I'm not claiming any of this is official Catholic Church teaching. It's just what I've picked up by hanging around for 50 years or so. If you know your medals, you don't need to read this. If they're a bit of a puzzle to you, read on.
I've finally gotten our local religious goods store to carry some scapular medal centers. Scapular medals are often worn - as medals - by people who want to wear a scapular but don't want the cord or ribbon showing at inopportune times. We're specifically talking about the brown (Carmelite) scapular here, as that's the most common one - and the only one I know of that has a more-or-less official medal equivalent.
A scapular medal has an image of the Sacred Heart on one side and an image of Our Lady of Mount Carmel on the other. I'm trusting that most people who make rosaries know an image of the Sacred Heart when they see one (although I'll be posting one farther down this post, for a different reason). But Our Lady of Mount Carmel can be a bit tricky to recognize. Some centers will help you out by supplying the title, either in English or Latin (or French, or Italian...). If you see "Carmel" anywhere in the inscription, you're almost certainly looking at Our Lady of Mount Carmel. But some don't give you a label. Recently, in the course of doing my regular photography for my Etsy shop, I managed to get a really good picture of that often too-tiny-to-decipher image:
(A note that I have seen at least one statue of Our Lady of Mount Carmel where Mary and Jesus were not wearing crowns, but I've never seen such a medal or rosary center.)
Another medal that often shows up as a rosary center is the Miraculous Medal. This can take many forms and some of them are quite lovely, such as "open gate" and "open arbor" styles. There are two things necessary to qualify as a Miraculous Medal center.
First, you need Mary in this pose (No, Mary doesn't need to be standing on her head... I've learned that Blogger rotates my photos sometimes because it's not "compatible" with my camera. It often turns them sideways, but this is the first time it's flipped one):
Then, on the back, you need this inscription (thankfully, this one's right-side-up):
Here's an example of a smaller, "open" style Miraculous Medal center:
The reason this can get tricky is that you sometimes have the front of the medal:
But something else on the back (this one is an image of the Sacred Heart):
So, in this case, you don't have a Miraculous Medal, even though this closed oval is the version of the image that we're most used to seeing there. What you have is an image of Our Lady of Grace, with the Sacred Heart on the other side.
Here's another version. The front of this center has Mary in a perfectly valid pose for a Miraculous Medal (turned sideways):
But on the back, we have - Mary's back:
So, again, we have Our Lady of Grace, but not a Miraculous Medal.
There are a lot of other rosary-center images we can talk about, but these two are the only ones I can think of that incorporate specific, more-or-less official medals. Have I forgotten any?