A piece of D'ni technology, actually a stone charged with with a great aount of energy. It is combustible but can be extinguished by water. Polished well, it produces bright light and can be used as a light source in special lanterns.
Gehn had access to fire-marbles while stranded on Riven. While he couldn't use his books, he found out that fire marbles had such an enorous surge that could suppress the variance and hold a Descriptive Book in a stable matrix. After some modifications to the fire-marbles he managed to link to Age 233.
Gehn also created a power system on the top of the Great Golden Dome that powered the rotating domes around Riven; it required fire marbles of various colors to be placed in the right combination on a grid, each representing a rotating dome on its specific location on each island.