It is more than likely anodized aluminum. There is a process for removing the anodized coating so you will have clean aluminum to carry in but it is dangerous so I will not mention it here. If you want to test it cut a small corner off and put it in a small container of hydrochloric acid (HCL) and let it set for a few days, aluminum can react violently with HCL so do not heat and do not close container tightly because of the gasses that are produced. Please do not try this at home.