I know these beads from a Thai context.

The literature on ancient beads tells us that joined agate beads were made in antiquity. Either these were made to imitate banded agate beads, or to imitate chemically-decorated stone beads.

The current Thai beads likewise imitate a class of Thai decorated agate beads (that are sometimes compared to zi beads). I have inquired of the (two) experts I know as to whether they believe there are modern interpretations of such beads (with no opinions offered).

About three years ago and over several months, I bought specimens of the recent joined beads, from eBay offerings (where they were misrepresented as "ancient"). I got enough to make a necklace. I bought two additional beads that are so long they do not easily fit into the design. My photos of these beads are stored on an external drive, and so are not at-hand.

I believe the stones are reddish carnelian and black agate. I think the pieces are glued together as flush surfaces, but one piece has a carved recess on the ends (creating a thin trench)—and this is filled with a hard white compound, to simulate the decorated white lines of the original beads. This also disguises the joins. However (as you probably discovered), it's possible to feel these joins by passing you fingernail lightly along the length of the beads.

I'll try to shoot a new photo, since this will be easier than looking for photos from ca. three years ago.

Jamey

P.S.—I had intended to come to London in June. But, alas, you know what happened. And I have been sheltering at home since March. I resent not being able to get out and see anyone; but I am resigned....