Metal covers can be made from Nickel Silver, Brass, Aluminum. Brass is probably used the most as it is economically priced and can be deep drawn easily. Nickel Silver is also called German Silver and is also used in a lot of jewelry. Steel should not be used as it will absorb the magnetic field that would normally be attracted to the strings. The various covers are plated making it difficult to determine what the actual material under the plating is.