Thank you all for your help with this necklace. I would say that Beadman is correct that the beads are more brownish than pure black. One of the spacers near the middle fades from almost black to brown, if that is a clue to the material. The spacers have diagonal parallel lines which seem to have come from their manufacture. One dealer in Tucson thought it was soapstone but couldn't say for sure. Another dealer thought the shape of the spacers and the relative largeness of some of the bead holes suggested Hellenistic influence. That's all I have been able to gather so far.