From the strand pictured below (sold to me as vulcanite trade beads) I tested two random beads, one black and one red, by reflectance infrared (IR) spectroscopy - and they are definitely not natural rubber based polymer. Their "fingerprint" was closest to PVC. I took two others of each color and tried to dissolve them in several solvents. One of the red disks and dissolved completely in MEK (methyl ethyl ketone), proving that it is not vulcanite or any type of crosslinked rubber. However the black disk that went into the MEK was not attacked at all. I will check that one with IR and see what I get.
As I examined the disks more closely I could see that there were different thicknesses, different shades of red, and slightly different diameters. At this point, the best I can say is that the beads are made from at least two different polymers. I'll see what else I can discover after I get back from Tucson.