Zone beads are another pretty large category of polychrome bead, though they only seem to be called zone beads in Asia. Zone beads are any bead with one or more lines around the centre, running perpendicular to the perforation, such that the bead is divided into two halves of equal size. In order for this to work, the line must connect to itself, forming a circle all the way round the bead.
Generally, zone beads consist of a dark core with a light (often white) line running down the centre. Sometimes they have two lines parallel to each other, and on occasion they have three lines, all parallel to each other, and the middle line is a different colour (often red). This basic type are fairly common in the Middle East and South Asia, though some have been found in European regions occupied by the Roman empire.
That being said, if a line dividing the bead into two equally-sized halves is the only criterion, there are many, many other styles of zone bead in the world. In Europe, I’ve seen the typically dark core and lighter line pattern, but the core is a translucent cobalt blue and the line either white or yellow. I have also seen reticella lines dividing the bead. I haven’t really seen these styles outside Europe.
Some zone beads may also have other designs on them, if a single, central dividing line is all that’s required for a zone bead. Sometimes eyes will dot the two halves, or even another line that zigzags or curves between both halves.
Some zone beads are though to mimic stone beads, like onyx or agate. Others are clearly not meant to look stone-like. Many are folded, with the glass being arranged beforehand and folded around a metal rod. Some are made by winding the core around a rod, then winding the dividing lines around the core. A few, like the one to the left, are folded and then pierced to create the perforation. The lines, like eyes in eye beads, may be marvered into the core or may sit on top of it, creating a bumpy surface.
Zone beads are fairly common in South Asia, but I honestly haven’t seen very many zone beads outside of South Asia. Some are found in Southeast Asia, some in the Middle East, some in Europe. The type is certainly recognized across Asia and Europe, but the style isn’t nearly as common as eye beads.