"Riddle Me This!"

I asked my wife to look at jervistetch's answer and then give me a hint without giving it away. She said my paragraph 2 rather than 4 was closer to the answer. So I stopped mulling about shelfpaper and went over the shell bit again. I remembered seeing an old jewelry box adorned with shells. I found some photos online of such jewelry boxes, many with shells large enough to hide a diamond it yet small enough that a person might not think to look there. I could ask if that's right, but my wife already said, "You got it!" so I'm pretty sure that's it.