Chromastone is a Crystalsapien (a combination of crystal- and -sapien which means "intelligent being made of crystal") originally thought to be from the planet Mor'Otesi (Turkish word for ultraviolet). The planet happens to not exist; the events in Ben 10 Alien Force: Vilgax Attacks being non-canon for Sugilite, who is the genetic donor for Chromastone, is the sole Crystalsapien and the guardian of Petropia(Diamondhead's Planet).
Chromastone is Cyclops alien made of an extremely durable crystal, and has several shards of magenta crystals. He is almost indestructible with his crystal skin; has a great strength and an ability to fly; he is able to absorb kinds of energy like a conductor and channel it into ultraviolet laser blasts or less-intense natural light, he can also allow it to pass harmlessly through his body via refraction, or can be blasted back at the foe; he can seemingly produce blasts without having to absorb energy, though it may be due to absorption of radiant energy like sunlight, or streetlamps or just ambient energy; however he is unable to absorb energy when not expecting it or is too weak to do so. He first used Chromastone to stop the zombified schoolgirls in "All That Glitters".
During the episode "Vengeance of Vilgax: Part 2", Chromastone was destroyed by Vilgax, but the alien's remains reformed into Diamondhead, the reason for this is connected to the fact that Sugilite is the only one of his species. He hid himself until the time came for him to restore Petropia, which was shortly after Vilgax's destruction of the planet. It is believed that Sugilite is the guardian of the Petrosapiens, and that he was created as a backup system for the planet in case the planet was destroyed. Although Chromastone becomes accessible again after the events of "The Secret of Chromastone", Ben didn't use him until the pilot episode of Ben 10: Ultimate Alien, to fight Bivalvan, being very surprised that he still had access to the form.