Stone-sickness
Stone-sickness was a disease that afflicted flight-rocks. It made them lose their buoyancy, and, in severe cases, flight-rocks would crumble to dust. Sky ships infected with stone-sickness were prone to dropping out of the sky, which led to the end of the First Age of Flight. The New Sanctaphrax rock became infected, causing it to sink downwards, and the Stone Gardens no longer produced new flight-rocks.
Cause
The cause of stone-sickness eluded the inhabitants of the Edge for centuries. Various Edge-dwellers had countless theories on its cause and its cure. Some said that it had been brought by the Mother Storm from Open Sky, while others said that the disease was due to Cloud Wolf perishing inside the Mother Storm and infecting her. Another theory was that stone-sickness was simply a punishment on those Edge-dwellers who refused to give up their evil ways. The two most important theories on the cause and cure, however, were the theories propounded by the Librarians Academic and the Guardians of Night. The Librarians believed that the cure for stone-sickness had to exist somewhere in the Deepwoods and could be discovered through study. The Guardians, on the other hand, believed that the contagion responsible for stone-sickness actually came from the Deepwoods, and that the cure would come from the sky. Specifically, they thought that the disease would be eradicated when a bolt of sacred lightning from a Great Storm struck Midnight's Spike. The Guardians and Librarians viciously fought against each others' beliefs, and the Guardians eventually cast the Librarians out of New Sanctaphrax. The Guardians' theory was eventually proven wrong when a lightning strike destroyed the Tower of Night, but the Librarians too were unable to find a cure.
The true cause of stone-sickness, it was revealed hundreds of years later, was the Gloamglozer. The disease was born along with the evil demon in the Ancient Laboratory, and, once the Gloamglozer was banished from Sanctaphrax, stone-sickness began to spread. Stone-sickness initially passed unnoticed, and incubated in the flight-rocks for decades. It was not until the birth of New Sanctaphrax that stone-sickness finally started to produce the telltale symptoms of crumbling and loss of buoyancy.
Cure
Hundreds of years after stone-sickness first appeared, the disease was finally eradicated. The Gloamglozers inhabiting the ruined city of Old Sanctaphrax were killed by the Immortals and the disease died along with them. The moment the last Gloamglozer was destroyed, the rocks began to heal. This was because a storm came from Riverrise carrying chine which healed the rocks.
References
Stone-sickness on the 'List of fictional diseases' on Wikipedia .