Here is a typical Idar-Oberstein stone bead, presumably made of agate per Frank's description. One side has a nice banded figure.

I have seen the more opaque jasper faceted rounds such as shown in the IP, but I don't have any in my collection at the moment. However nearly all the Idar-Oberstein beads I've seen were clearly the more translucent agate stone.

Another comment - many people confuse the almost identical-looking, dark red-brown glass faceted round beads from Bohemia with the stone beads from I-O and they are sometimes found together on the same strands.