I suspect that all ferrules are brass because they need to be deep drawn and brass is an ideal material for this process. I further suspect that most of these get plated with nickel (bright and shinny) or painted (usually black). It may be possible to remove the paint from the painted ones to get down to brass. Brass looks great when it is shinny. When exposed to the elements it turns a dull brown and therefore why they are painted or plated.

When someone tries this please report back here on the results.