Rubyman is a heroic member of the Rock People who turns into a ruby rock. He is often associated with the two Jewel People, Diaman and Amberman. He is a heroic character who will willingly sacrifice himself for the greater good, even of those he has just met. He has the ability to heal others and restore their energy. Because of his mechanoid nature, Rubyman does not need to breath oxygen. Like most Rock People he is probably immune to the effect of acid and magnetic fields which adversely affect metal mechanoids. He carries a Hard Spear weapon. He stands 6 feet 6 inches tall.
Machine Robo: Revenge of Cronos
Rubyman first appeared in Machine Robo: Revenge of Cronos episode 35, "The Secret of the Jewel People". In the episode Rubyman was trapped in Devil's Cave and sent Magnarock for help. Magnarock returned with Reina Stol, Pro Truck Racer, Rod Drill, Rom Stol and Triple Jim, who freed him. Rubyman used his powers to heal an injury to Pro Truck Racer. When the cave collapsed Rom Stol summoned Vikung-Fu to break through the cave wall. Outside the cave they were confronted by Gandora forces lead by Diondora. She promised not to harm the other if Rubyman went with her. Although Rubyman agreed to go for the sake of the others, Blue Jet arrived with reinforcements and helped them overcome the Gandora. Rubyman returned with the others to the Battle Base, where he was drawn to the Dragon Helm. 
In episode 43, "Mighty Fortress, Heavenly Sword - Jet Don't Die!", Rubyman was among the commanders of the Battle Base when it attacked the Varigale-X.
Rubyman last appeared in episode 44, "Cronos' Last Battle". During the final battle with the Gandora Rubyman informed Amberman that the artifacts he had protected were in the hands of the Gandora. Rubyman was among those who celebrated when the Hyribead healed the planet.
Note: Some English subtitles of the series refer to Rubyman as Gem Machine Ruby and is a member of the Red Stone People.
Machine Robo: The Running Battle Hackers
- Bandai Machine Robo: Revenge of Cronos MRR-8 Rubyman (1986)
- A new mold. Turns from robot to ruby. Comes with his Hard Spear weapon.
- This toy was released as Rock Lords Flamestone.