So, I was curious of this as well. Here's what I found:

"Why is glitter so addicting? Because it is mesmerizing, shiny, happy and oh-so-magical looking. It adds a crystalized dimension to anything it comes in contact with, whether it be a handmade gift tag, a shadow box or even your eyeball (ouch!). It dates back many centuries and comes in all kinds of grades, sizes and qualities. Germany is known for creating a high-end version made from crushed vintage glass!

Glitter is made from ultra fine particles of plastic films and metallic foil. When used to accent an otherwise "flat" surface glitter will give off a sparkly brilliant sheen that will stand out strong and make people gasp in glee."

http://www.craftychica.homestead.com/glitter.html was my source.