Did a theory on the Roger Pirates a long time ago. Can't remember exactly what it was for, but it had to do with Jewelry Bonney, and the Roger Pirates. But anyways, I just remembered something I had brought up in the theory.
A Shank is the part of a ring that holds the Gem in place.
Now that I think about it, Gems are often called "Rocks" in slang terms in reference to Jewelry.
Do what you will with this information.