After a long struggle and several trials, I finally succeeded to make a "Ram's Horn Shell Bead". This bead was remarkably designed by Sharon Bateman and the explanations appears in the book "The art of beaded beads, exploring design, color and technique"(Jean Campbell) p.86-89.
It was a very challenging project in which I used Matsuno and Toho 11' seed beads and it took me about three hrs. Whoever intends to try it, please remember it's important to start the spiral in the correct direction, otherwise it's a mess.
The colours I used for this beaded shell won't suit a jewel (or maybe it can?! I don't see how...), but it can just stand as a beaded sculpture.