In many popular versions of Cinderella, including Disney’s, our heroine wears slippers made of glass when she attends the ball given by the Handsome Prince. The earliest known rendering of the Cinderella story is a 9th century Chinese version, which makes perfect sense considering that culture's appreciation for small feet. In the earliest telling, the slipper was made of gold; in other versions, it’s not a shoe at all but a ring or a diamond bracelet (one of a matching set) that leads prince charming to his elusive bride.
It was the French author Charles Perrault who wrote down the version of Cinderella (in Stories or Tales from Times Past, with Morals: Tales of Mother Goose, 1697) that introduced the glass slipper, the (fairy) godmother, the pumpkin-transformed-into-a-beautiful-coach, and the witching hour of midnight as the deadline on all of Cinderella’s finery. In other versions, Cinderella (Cinderwench, Cinderbreech, Cindertail) is helped by her dead mother, or some agent thereof; a domestic animal (cow or goat), a magical fish, or white doves perched in a tree of hazel growing on her mother’s grave. In Perrault’s telling, Cinderella forgives her evil stepsisters at the end of the story and finds them husbands in the royal court; less forgiving versions (like the one by the Brothers Grimm) include the stepsisters cutting off parts of their feet to try to fit into the slipper*, and having their eyes pecked out by the doves at the end of the story.
There is some controversy over the idea of slippers made of glass, and some have suggested that Perrault, transcribing an oral account, accidentally substituted pantoufle de verre , glass slipper, for pantoufle de vair, fur slipper. This seems plausible, especially considering vair referred at one point to squirrel fur, which was used, like ermine, to trim robes and dresses. Unfortunately, this theory doesn’t hold water; vair, from the Latin varius, varied (as in variegated fur), is a medieval word, and was not in use during Perrault’s lifetime.
So we are left with the conclusion that Perrault’s use of glass slipper was an intentional embellishment of the story. This has led to all sorts of speculation on the symbolism behind glass—a representation, perhaps, of purity or chastity. Certainly a glass slipper (or a golden one) wouldn’t give the way one of leather or cloth would, so it would be less likely to fit the wrong person. There’s actually a fair amount of glass in fairytales—Snow White’s glass (or crystal) coffin, magic mirrors, crystal goblets and crystal balls, shards of glass and ice in The Snow Queen, and one version of Bluebeard, in which the key to the forbidden room is glass. In any event, the glass slipper seems to have captured our imagination, and the image is an enduring one.
* From the Grimm Brothers version: “ Cut a piece off,” said her mother, offering her a knife; “when you are queen you will not have to use your feet much.”
Master Villain adds, "Wasn't glass also a hot consumer item, like plastic in the 60's? It would make the slippers seem so much more valuable. This goes with all the other glass oddities...."