Set up the trick with a small change purse containing two real coins and the expanded shell coin. Remove one coin and the shell without letting the audience see the third coin in the purse.
Show the audience the front of the shell coin and the same face of an actual coin of the same value. Hold both coins between the finger and thumb of one hand so they overlap slightly with the shell coin in front.
Place the coins in the palm of the other hand, taking care to keep them in contact with one another with the shell coin on top.
Shake the coins slightly while claiming that you will make one of them disappear. When the shell coin is almost completely covering the real coin, close your hand.
Feel the shell coin slide over the real coin in your hand. Perform a fake incantation or other patter.
Open your hand to show there is only one coin in your palm.
Claim that the missing coin still exists, but has only become invisible. Pretend to pluck the coin out of the air and show it to the audience (who, of course, can't see it).
Pretend to place the invisible coin on top of the change purse and "rub" it with your fingers to force it to pass through the purse.
Show the coin inside the purse.