They tumble OK, the rhyolite undercuts a bit leaving seams of Agate sticking up from the surface.
When they are nicely rounded to your satisfaction in the coarse take them out and grind the Agate level with the surface again then run them in 220 grit for the usual time to remove any grinding scratches. then process through the other stages as normal too.
Half T eggs when tumbled can be used to make novelty turtles if that floats your boat, they really do look like turtle shells.