Diamondhead is a Petrosapien (Petro- referring to rocks, sapien referring to an intelligent being; Petrosapien would mean intelligent rock) from the planet Petropia (a combination of the Greek Petro- and topos, forming a word similar to rock place). Ben first transforms into Diamondhead in "And Then There Were 10" when he needed to fight Vilgax's giant robot.
Diamond Head is a crystalline alien whose body is harder than diamond, making him invulnerable to most physical attacks and allowing him to cut and slash through almost anything. He can also use his body to reflect light and beam-based weapons and has chromastone's power to shoot laser beams also now his diamonds are more powerfull then before which makes him stronger.
The one drawback to Diamondhead's crystal form is that it can shatter if exposed to sufficiently strong sonic vibrations or if smashed brutally with any blasts. While Petrosapiens can regenerate limbs, there is a limit to how much damage they can recover from in a short specific while. Strong sonic bursts usually do him in. This is shown in Ben 10 vs. The Negative 10 Part 1 when he crumbles when facing against the screech attack from Animo's giant super bat, or since his nose seems to resemble that of a star-nosed moles, and his rat-like tail "Mole-Bat." And in Ken 10, when Diamondhead is facing off against Kevin 11 000, he gets hit with Benwolf's sonic howl.
Petropia is a crystal-covered world, and its inhabitants are silicon-based lifeforms. The majority of its inhabitants live in underground kingdoms. The planet was destroyed by Vilgax with some assistance from a Petrosapien named Tetrax, who unknowingly helped Vilgax destroy his home planet.