Half Dollar Shell

Most coin magic books (Roth, Coin Magic, Piedrahita,...) feature effects using a shell.

However, I firmly believe that the shell is something you should only start using once you've gained proficiency with regular coins. You can do almost anything with regular coins. I always bring up Gea as an example; he rarely uses anything but regular coins.

And beyond that, if we're going to dive into the world of gimmicked coins, I'm much more inclined to recommend a copper/silver coin over a shell to begin with.

But circling back to the shell, as I said, almost all books feature at least one effect, and Ricardo Vizcarra's notes are exclusively focused on the shell.

Tangopedia's DVD has some great ideas for using the shell.