Actually, Cinderella's slipper was originally made of squirrel's fur. The French word for squirrel is very similar to the word for glass (please don't make me go look it up, but I will if you ask me to, 'cause the book's on the kitchen table, only partially buried) and thus the story changed in the retelling upon retelling upon retelling.

I mean, really... wouldn't soft fur make a nicer slipper than cold, hard, (and potentially sharp and blood-letting) glass?