Rock Human

The Rock Humans (岩人間) are a presumed race of mysterious beings that serve as antagonists in Part VIII: JoJolion.

Summary
The Rock Humans are a race of idiosyncratic humans whose original forms take on a "rock-like" composition. In this form, most biological activities come to a halt and they're able to endure high and low temperatures, dryness and other things in nature. It is unknown how long they can become like rock, but when their biological activities resume, they require at least a small amount of skin respiration, so they will suffocate in areas where there is no air. They must try to stay awake and active as much as possible, as the moment that they lose concentration they will turn to rock and hibernate for at least a month.

While they are generally represented as humans, Iwasuke, a dog of unknown origin, also appears to have similar abilities and is described as a rock dog.

In the manga's latest chapters, the appearance of two identical twins who are presumably rock humans present an interesting facet of their physiology; the elder brother appears to be missing a large section of his right hand and most of it's fingers, with the wound in a permant rock-like state.

This could mean one of two things - either he consciously keeps it in a rock state as a way of preventing blood loss and infection or the disruption of his hand has caused localized rock-hardening usually observed in Rock Humans who die and lose their sentience. or alternatively it is a unique deformity or feature of this specific individual.

Abilities and Powers
Rock Skin: Each Rock Man has the ability to shift their outer appearance from a normal human into a rock-like state and vice versa. In their rock form, they are able to camouflage into their surroundings, and improve durability to some extent. The ability to stay human requires some type of focus however, as Aishou describes his rock form as something he was born with, but unable to control at a younger age. Yostuyu, who is presumably more experienced in controlling it, is able to revert to human, rock, and a hybrid where he can still move despite having a rock-like appearance. After a rock human is killed, they lose control of their sentience and their body turns back into a rock.

This form can be used advantageously as camouflage or in Yotsuyu's case, can forgo the need for accommodation or food. By transforming into a rock in grassy or secluded areas in or outside of the city he can essentially live without the need for housing or other oridnary expenses which allows a sufficiently experienced Rock Human to concentrate his time and resources on pursuing other things.

Immortality: The Rock Humans are stated to have long life spans, though to what extent is unknown.

Trivia

 * The concept of the Rock Humans is similar to the Pillar Men featured in Part II: Battle Tendency.
 * It is noted by Norisuke Higashikata IV that the Rock Humans may lack genitalia, as the remains of Yotsuyu Yagiyama did not appear to have any. However, Yotsuyu's remains had deteriorated when they checked, and Aishou Dainenjiyama was shown to be in some sort of romantic relationship with a human woman, possibly implying Rock Humans do in fact possess reproductive organs or at least that they experience romantic feelings.
 * Although their motives are unclear, at this point in the manga the 'Rock Humans' are the collective antagonists of JoJolion.