As the clamshell term suggests, this finding has two small cups that look like a clamshell, and the knot on the end of the beading cord rests inside of these two cups that you then close up around the knot to secure the end. There is a little hook on the end, and this is what you can attached to a jump ring or also a clasp.
For example, here's a necklace I have that uses clamshells on the end.
I tend to use either crimp beads or clamshells on the end of my beaded pieces. Which one I select can depend on my mood but also on what I'm making. However, I have to admit that I find clamshells much easier to work with and longer lasting than crimp beads.
Finishing touches to a jewelry design can be really small sometimes but make such a difference in the overall look.This is one reason I have started to think more about the type of metal finish that my clamshells and other findings have and either purposely try to mix up the metals (when I'm going for an eclectic style) or try to match them (when I'm making a piece that is classic in design).
Clamshells, whether they are silver or gold tone, come in different finishes: bright, antique, black. This is something to think about when you are getting your findings together and before you sit down and start creating your strung jewelry pieces.