The Firemarble dome is one of Gehn's more ingenious, albeit brutish, inventions. The domes, which were built on Riven, served two purposes. First, they shielded the Linking Books they held from the elements, as well as the Moiety. They also provided power to the Linking Books, which were faulty and unable to operate without an external power source. There were five domes in all, one for each island in the Age of Riven. These five domes were powered by a highly volatile and explosive kind of firemarble called a "power-marble." Power itself was provided by the Golden superdome.
Access can be achieved in 2 phases: Phase 1 is completed by using a kinetoscope-like device with a button meant to be pressed when the golden D'ni color symbol is seen. This sequence stops the dome from spinning and flips it over to its gold underside, initiating phase 2, which is a combination lock of 5 sliders in 25 notches, the code to which can be found in Gehn's Book Assembly Island journal. After setting the code from right to left and another button is pressed, the dome flips again, spins once more, this time with you in it, and the Linking book and pedestal are revealed at last. To get out again, just press yet another button on the floor.
There is one major exception to this process: The scope on Survey Island is also located directly above a pond where Gehn's semi-tame pet Wahrk resides. Sometime before the Stranger arrived on Riven, that Wahrk was acting a bit overzealous, and smashed into the metal support underneath that very catwalk where the scope was bolted. As this happened, one side of the wooden floor sagged to the left, thereby knocking the kinetoscope out of synchronization with its dome on the opposite side of the water.
Catherine and the Moiety were able to break into one of Gehn's domes and use it to power the Linking Book to the Rebel Age, Tay. Gehn was, as best as we can determine, unaware of this occurrence, though he quickly became quite aware of the fact that the Moiety were able to link into and out of Riven without his knowing about it.
It should be noted that the rain collection "bowl" on top of Gehn's office in Age 233 should not be confused with firemarble domes.