On the far side of the River of Fire are the underground Mines of the Jeweled Ones. They are reached through an underground passage that runs far beneath the River of Fire, and apparently unknown to the Jeweled Ones, also via a tunnel from the Lair of the Reptile-Men.
The mines are worked by human men (the grown male offspring of the Cave-Women that are not selected for other tasks.) It is rare for a man to try to escape from the mines, since they have been raised since infancy by the Jeweled Ones and know no other life.
The Cave-Women will occasionally hazard the trip through the Lair of the Reptile-Men to go in search of mates, since the only other men they know of are the jungle savages.
The mines are but one part of a complex of industries in the second ring of the Island in the Hidden Sea of Fire.