It is not possible to give a single description of a "native American bead" because so many different types were made by the natives themselves or obtained in trade.

Pre-contact beads were manufactured from many types of shell, from bird bones, from animal bones, from dried berries, from stones and even from copper.

After contact with Europeans glass beads were traded to most tribes; at first these were fairly large "pony beads", but later smaller "seed beads" became popular. Beadwork can often be approximately dated by the size and colours of glass beads used.

If you are interested in the subject I would recommend a book entitled "Beads and Beadwork of the American Indians" by William C Orchard, which details all the many types of beads used by native Americans.