They are plenty symbolic, which is all I found online at first, but then I found this:
Shell:
The homes of the small creatures of the sea, appears in many forms and contains the energies of the god of the sea. These vacated homes have been used to provide boundless growth in all areas of endeavor. They can be used to stimulate intuition, sensitivity, imagination, and adaptability. One type of shell, the abalone shell has also been used to enhance smudging activities, bringing the energy of continuity to the environment and instilling a stable atmosphere. It has been revered by the ancient tribes throughout the world as a magical gift from the sea - one which can stimulate fertility of both the mind and body and can protect against un-cooperative attitudes and actions.
The credibility is a little shakey being from a jewelry site, considering intentions to sell et cetera, but it was at
http://www.keystonejewelry.com/id24.htmSymbolically, it seems to hold the same value as the chalice in Druidism.