Gears today are made of many different materials, fiber, plastic, composits, and all different metals (brass, aluminum, steels). Each has their own application, with those made of the various steels being the strongest, plastic or stainless steel used any place it might come into contact with "goods" for human consumption (S.S. used if the load is great, plastic if it is a lesser load). Those applications 'cross' each other, sometimes, as long as the material the gear is made of is strong enough to withstand the task it is being used for. Your composit gears will run quieter than steel gears. They are probably not original to your lathe, but, gears that were replaced or added later in its life. They won't last as long as steel gears, but will "do the job" for awhile.
