You may see all sorts of beads labeled as “opalescent,” but a true opalescent bead is one that has qualities similar to an opal. The bead should be semi-opaque, cloud, and iridescent.
The term now is sometimes applied to beads that are milky or cloudy, even if there isn’t an iridescent look to them.
You know you have an opalescent bead, when you hold the bead to the light, and the light is reflected in unusual ways.
The first opalescent beads were most likely white, but you can find colored opalescent beads, as well.