Polystyrene There are different sorts of polystyrene differing in densities. Ours are of medium density so will easily hold pins and can be cut into using a thin bladed sharp craft knife. Polystyrene – egg – 7cm £0.85 Polystyrene -ball – 4cm £0.25 Polystyrene -ball – 7cm £0.80 Polystyrene -disc – 8cm £1.00 Sale! Polystyrene -diamond £1.00 £0.80