From Halopedia, the Halo wiki
Incendiary gel is a radioactive composite created by the Covenant from the energetic meta-materials and strained-bond lattices present in the infusion matter developed by the Forerunners, and present throughout their installations. The Covenant's incendiary gel is considered an inferior copy of the Forerunners' own perfected forms, though is widely employed across many different weapon systems.
The adhesive forms of incendiary gel found in Covenant firebombs are considered stable, yet hazardous nonetheless, and contain caesium. In this application, incendiary gel burns at approximately 2200oC, and will not burn underwater.
Incendiary gel is utilised in a wide array of applications across the Covenant military, though perhaps the most famous is that used in the assault cannons wielded by the Mgalekgolo. Some assault cannons fire their incendiary gel in individual bolts or magnetically-guided streams of gas. Incendiary gel is also used as the filler in Covenant explosive ordnance, including the fuel rods fired by various fuel rod guns and fuel rod cannons, and as the primary filler inside the Orgudam Workshop Firebomb employed predominantly by Jiralhanae forces.