Chlorinated plastics (PVC, vinyl, pleather, Sintra, Kydex, etc...)
Polycarbonate / Lexan
Polyethylene (milk bottle plastic)
Coated carbon fiber
Pressure treated wood
If you don't know exactly what a material is, then assume it is not safe to use.
Lasing any unidentified or prohibited materials puts you and others at risk and can severely damage the laser cutter. Doing so will result in revocation of membership and/or authorization to use the laser cutter.