Does an envelope have to be "paper"? or can it be...?

does an envelope have to be paper? or can it be like a shiney metalic plastic paper, i dont know how to explain but you know fro school when you where in elementary and you had the shiny mettalic folders for school. that go inside of prongs. they are paper like cardbordish but thin. you know what i mean? i cut it and made it into an envelope its really cute its shiney silver with like shiny stars that turn rainbow when you move it. and i glued white paper so the P.O can read the adress. do you think it will b ok or it wont process?.. ive made home meade enelopes before but i just colored paper and put stickers on it.