Locacaca

The Rokakaka (ロカカカ) is a mysterious species of plant featured in JoJolion.

Summary
The exact nature of the tree and its fruit continues to be one of the central mysteries in JoJolion, alongside the circumstances which lead to the creation of Josuke. It is first mentioned to have existed when Tsurugi explains to his grandfather and Josuke why he trusted Yotsuyu Yagiyama; he had observed the man feeding a dog afflicted with the stone disease a fruit, and it seemed to have been cured as a result. Although the two are skeptical, Josuke is the first to see what appears to be a fruit disappearing into the ocean during his fight with Yagiyama, just as the man dies and crumbles into stone before being able to tell him what the fruit was.

Norisuke IV uses the abilities of King Nothing to automatically track the fruit, and Josuke discovers the fruit's scent happens to be lingering on Tsurugi's father, Jobin Higashikata. Josuke, Tsurugi, and Yasuho resolve to obtain the drive record data from Jobin's car in order to find out where he has been in hopes of discovering the location of the fruit's tree.

Yasuho and Tsurugi discover a Rokakaka tree in the possession of Yotsuyu's associate Aisho and it is then that its nature of "equivalent exchange" is demonstrated when an elderly man missing a leg eats a Rokakaka fruit and regrows his lost limb, losing his eyes in the process after they turn to stone.

The nature of the fruit and its miraculous abilities have made it the subject of many people's interest, including Yoshikage Kira for its abilities to assist in the treatment of his ill mother Holly, and it is noted that after destroying several trees, he kept one in his apartment for his personal use. The Rock Humans also seek out the fruit, as they were the ones illegally importing and selling it in Japan until Kira and Josefumi Kujo stole a branch from them.

Yasuho later researches the Rokakaka, and finds that it originates on the island of New Guinea, where it was known to the natives as recently as the 1930s. At certain times of the year, men would go into the mountains to harvest the Rokakaka fruit, which doesn't come from a tree but rather a perennial plant that has one to three fruits growing from it. Finding a Rokakaka was rare, even for the native Papuan peoples who lived there for 50 thousand years. But there were stories that women suffering from infertility ate the fruit and became pregnant, but their breasts gave sand instead of milk, and the elderly who had lost their teeth through the years swallowed the fruit and grew a set of stone teeth to replace them. However, all reports of the Rokakaka disappeared after the 1942 invasion and occupation of New Guinea by the Imperial Japanese military, until she, Tsurugi, and Josuke came across the Rock Humans in possession and searching for the Rokakaka in the modern day.

Equivalent Exchange
The fruit of the Rokakaka tree follows the same principles of "equivalent exchange" as the grounds of the Higashikata estate; they are capable of granting anyone who eats it "something" by taking "something else" from them. The most faithful example of this concept is when the old man uses the Rokakaka fruit to regrow his lost leg in exchange for his eyesight, both of his eyes transforming into stone and breaking apart soon thereafter. Apparently this process can be repeated as long as someone has a fruit, continuously sacrificing random parts of their own bodies in order to gain back what they had given in the previous instance.

This effect could explain the missing pieces of Holy Joestar-Kira's brain, seeing as Karera noted that her son Yoshikage Kira had personally kept a plant for use in treating his mother's illness.

Yasuho's research into the fruit of the Rokakaka shows that the equivalent exchange provided by the fruit takes the form of part of the afflicted's body turning to stone. Infertile women gave birth but their breast milk turned into sand and the elderly grew back teeth made of stone in order to eat again.

Miraculous curative powers
Perhaps the most intriguing and relevant ability of the fruit to the storyline is the affect it has on those afflicted with the stone sickness; namely the Higashikata family, the Joestar family and the dog Iwasuke. As shown in the case of the latter Yotsuyu was able to 'cure' it if the stone sickness by feeding it a piece of a Rokakaka fruit.

The abilities of the fruit and their correlation to the rock humans and stone sickness make them a constant source of interest to the antagonists of JoJolion, which motivates the conflict between them and the allies of the amnesiac Josuke.

Trivia

 * The Rokakaka fruits bears a slight similarity to Cucumis metuliferus in terms of outer appearance.