How to make a bottle cap necklace?

OK so my friend came to school one day this year and was wearing a thing called a bottle cap necklace.She explained what it was how she had a stretchy material choker-like on her neck(which she later on gave me and im wearing right now)but she never told me where she got it and i was wondering if any of you know where i can get some more stretchy choker things in different colors.And how she tied a little washer(the tiny donut-shaped things)to the string and hot-glued a thin magnet to the back of a coco cola bottle cap.The bottle cap had a little design and had a slight bit of glitter but it was fillled up wit this clear substance and it was really smooth does anyone know what its called and where i can find it?So thats basically what it was and the magnet sticked to the washer and could easily be pulled off and switched to another bottle cap with a different design.She said her cousin got it from her school and gave it to her but i wanna be able to make mine by myself so what would i need and any ideas for designs or anything else?