Why didn't Cinderella's glass slipper disappeared when all the other things dissapeared at midnight?

I'm not positive, but I think it's because the glass slippers were never anything else. Like, originally,the pumpkin was an onion, or something like that, the horses were mice and her beautiful dress was just rags. But the glass slippers were real, they weren't anything other than glass slippers. The other things were just the items of a poor girl disguised as those of a rich girl. But before the glass slippers, there was nothing. I don't know, but this is what I think.