Small victorian brooch with pearl

This is a small victorian brooch I picked up some time ago on a flea market. It has a c-claps and t-hinge and its about 1cm x 1cm. I seem to find more and more of this kind of brooches from around the 1880's or early 1900's. I believe its made of rolled gold and the middle part is some kind of other material, it seems to be copper. On the front you can see some of the gold has worn off from the middle part. This brooch is nothing special but I do love it.
I think its weird its made up of two different materials. Can anyone explain the reason to me?
Thanks a lot!