Try texturing the backplate prior to constructing the pendant or pin
or earring. You can use a rolling mill and patterned brass, handmade
paper, patterned steel, wire, lace, sewing thread, sandpaper, fabric,
textured wallpaper - anything that makes a close, interesting pattern
on the metal. Simply make a sandwich of the metal to be patterned,
the stuff to print on the metal and, if needed, another piece of
metal to protect the rollers on your mill. Or use a couple of stamps
or a nail set - anything to make the back not shiny. I usually
texture both sides of the metal at the same time. Then the front
won’t show abuse either.
If you don’t have a mill, try scotch-brite, scotch stone, swirled
application of sandpaper - something to make it not shiny. These
applications would be applied after completing the piece.
Not too surprising, all of these textures are ideal for tumble
finishing. Save your hands and your lungs from buffing compound. The
last thing you do is set stones.
Your customers will love the idea that the back is finished with
their private surprise. Suggest that finger prints and scratches
aren’t pretty and this (pattern) won’t show scratches. Make jewelry
to wear, not to fuss with.