Hard Rock was a giant man made of living granite, the last surviving member of an ancient underground civilization whose inhabitants had been slowly turned to stone by the radiation given off of the glowing crystals they used as a light source. Hard Rock was discovered in his inert form by the Quest team while investigating the crystals, and he was taken back to Dr. Quest's laboratory for further study. In the meantime, Zartan, hoping to study the effects of the crystals' radiation on Hard Rock so as to reverse his own partially petrified face, awakened the stone giant using a sonic device.
After coming awake, Hard Rock broke out of Quest's laboratory and made for Zartan's lair, with Jonny Quest, Hadji and Race Bannon following. At the lair, Hard Rock initially allied himself with Zartan, until Scorpio tried to kill Jonny and Hadji. Quickly realizing that Zartan was evil, Hard Rock attacked Scorpio, saving the boys, who he told to run. Once they were free and clear he used his strength to bring the lair crashing down atop Zartan and Scorpio, killing them.
Due to his indestructible granite body, Hard Rock himself survived and made his way back to the Quest compound. He thanked the Quests for removing him from underground so he could be awakened once more, and explained how, when his civilization flourished, he was a warrior sworn to protect its people. But with all the other inhabitants of the lost city dead, Hard Rock reasoned the Quests were now his people, and he joined their team as friend and protector. His immense strength and high intelligence made him indispensable to them on their many adventures, and he quickly became good friends with Race and the boys. He ably assisted the Quests during their confrontations with villains such as Skyborg and the evil Dr. Zin.
- Hard Rock is not his actual name. His real name was never given, however he accepted Hard Rock as a nickname after hearing Jonny's comment that his body was "hard as a rock."
- Precisely what became of Hard Rock following his brief tenure with the Quests is unknown. It is possible that, given how many years he'd lain dormant, he eventually bid farewell to his friends and set out to see the world.