I have seen these all around the internet. They are silver bottle caps, which have been flattened. I punched out a 1" circle and covered it with an epoxy sticker and glued the whole thing to the center of the cap. I then added a magnet to the back and voila!
This one is using sweet stitches:
And this one uses everyday enchantment:
I have three more to show you, but the teachers haven't received them yet, so I'll share them after.
Thanks for looking :)