Hands of the Hills published a booklet on these beads in 1990, written by Duangporn and Steven Dunning.(ISBN 0-9628072-0-6)

There I read that they were likely made in what is now Thailand and Kampuchea in the first centuries AD. (I will not quote more, to avoid the risk of infringing copyright rules). Of course, this means SINCE then -- not that all the ones now circulating are that old, specially since manufacturing has been resumed in the XX century.