At first, like other answerers here, I thought "coquille" referred to the shell shape of the bead. But further research turned this up:

http://www.perlesandco.com/advanced_search_result.php?keywor...

Based on this, it would seem that "coquille" is perhaps the French term for "bead cap". I don't think it has anything to do with actual shells in this case.

A bead cap is basically what the name calls it - a "cap" or topper type jewelry finding that you add to a bead. For round beads, it can go on either side of the bead or even both sides of the bead. I think of it sort of like a hat. Mainly it is used for decorative purposes.

Bead caps come in a variety of sizes, designs, and metals such as gold, silver, copper, and brass. They are often shaped like a half circle, or sometimes they are cone shaped.

http://jewelrymaking.about.com/od/beadresources/g/And-How-Do...