Puka shells are pieces of a Cone Snail shell. Puka is the Hawaiian word for "hole" and refers to the naturally occurring hole in the middle of the shell fragment.
The shell of the Cone Snail is cone-shaped, and closed at the larger end. While the abandoned shell is tossed in the surf and coral rubble, the pointed end is ground into sand. The "puka" is formed when the minor apex of the large end is eroded by the same forces that wear off the point.
In the midwestern United States at least Puka Shell necklaces were a huge fad in the early 1970s.