Jerry,
I don't pretend to know a lot about pearls, but now that you know the name of this (quahog pearl), I suggest you do some googling to find out more. From what I read, you absolutely cannot drill these, as the outer part is much too fragile.
To see some pictures of these, and a bit about their uses in the past, and what some others have done with these in more recent times, you can take a look at one of the posts on this board, here:
http://www.pearl-guide.com/forum/showthread.php?t=121