A chimney thimble can be made from several types of metal, including cast iron, stainless steel, galvanized steel, copper and aluminum. If your chimney is actually used for burning then you want to get the strongest possible material for your chimney thimble, and a material that is fireproof. You may also want an insulated chimney thimble. The best material for this is either stainless steel or galvanized steel because it is strong, long lasting and fire resistant. It also comes in many different sizes and shapes that are available for most types of chimneys. Check with your manufacturer to be sure which one of the two is best for your needs.