When it spreads to the brain, the deterioration can result in violent insanity. In the very final stages of the disease, often years after the skin is completely covered, it starts attacking the body's internal organs, hardening them as it did the outer flesh. An infected person can survive like this for years, living in misery as one of the Stone Men. Long before that, in the mid-to-late stages of the disease, the infection will spread to cover all of the victim's skin from head to toe, leaving them in agony. Greyscale kills slowly, often taking many years to progress to a terminal state. The discoloration comes in later stages: Shireen was cured in early stages before the disease could spread beyond her left cheek, halting the discoloration that normally results. In later stages the hardened flesh becomes mottled grey and black as it dies, resulting in the flesh appearing to have the texture of cracked grey stone, or like grey reptile scales - hence the name 'grey scale'.

The pattern made by the hardened and splitting skin is similar to cracks made in thin ice, and in later and more severe stages look somewhat like patterns of scales. Greyscale makes the surface of the skin harden and calcify, but in the early stages it is still fairly thin and brittle, so the body's movement makes the hardened tissue split into a network of cracks.