The brass plate is very thin, and can be sanded a bit thinner. Specific gravity is around 8. The mother of pearl is pretty dense to, with specific gravity of about 3. I weighed the engraved piece above and it's one-half of an ounce.
Either piece will be inlaid under the finish, so it will be removing the same volume of wood, but of course the woods are much less dense, probably averaging about .75 specific gravity in the area we're inlaying.