Polystyrene

It may be in different shape (Crystal, Clash, Expanded - in this case, ...).
Polystyrene core, PS called crystal, is a hard and brittle, can be transparent or colored. Its mechanical and thermal properties can be modified by adding plasticizers or butadiene (rubber) into a Styrofoam shock said.
This material is very easy to transform, by injection or extrusion for example.