We find Marginella Shell Beads on our sites and they are a sea shell found along the east coast. I know I have found them on the beach at Myrtle Beach, SC. I also have a couple pieces made from Abalone Shell.
Here's 4 of them in this picture long with a Shell Disc Bead. The Marginellas were rubbed on one side until a hole was worn through and then strung on necklaces or hung from clothing or blankets.