These beads were first produced during the 1600's and 1700's for trade with the Native Americans. European glass makers produced huge numbers of them, which were traded for furs, food or even, sadly, land. The beads had certain characteristics. They tended to come in strands of about 14 inches, and were generally not of good or consistent quality. Most notably, they were usually strung on twisted grass. The type of grass and knots used to secure them are consistent.
Then, in the 1800's and 1900's, trade between the US and Africa became more active. White settlers bought back the Native American trade beads, and sent them to Africa. Once again, the strands of beads were kept intact, and used as money.
Now, with a more sophisticated African population and an American interest in "primitive' style jewelry, the beads have come back home again. I have found them in flea markets and high end bead shops. Depending on the style of beads, they can be had for as little as $12 a strand. Here's my latest find. I'll feel a little guilty cutting the old grass strand and re-stringing the beads to make a pirate necklace, but I'll be thrilled to wear a little piece of history.