From Halopedia, the Halo wiki

Smartmatter is a type of material often used in Forerunner machines. It is composed of self-assembling micro-machine blocks reinforced by energy bonds. Too much damage can cause the bonds to fail and the machine cells to lose cohesion, which results in the entire assembly eventually disintegrating into fine dust and hard light embers. Phaetons and the various Promethean automatons are made from smartmatter.[1]


  1. ^ Halo: Warfleet, Weapon-Ships - p.82-83