Grey nylon plastic with a subtle metallic sparkle (formerly Alumide.)
About this Product
This model was inspired by the "restraining bolt" used in Star Wars, by Jawas to prevent their droids from wondering off.
Multiple can be stacked, or a magnet (such as http://amzn.com/B000SJ61EC) can be glued into the bottom.
This model is hollow. A solid version is also available.