Petrusite is a powerful element made from a unique form of Higgs Boson molecules only found on the planet Helghan and in a few of its fauna. In Helghan's early years, it was used to supply Vekta and Helghan with energy. But when Scolar Visari gained power, he used Petrusite to create weaponry during the Second Extrasolar War.
The Arc Tower, VC5 Arc Gun, and even more weapons are powered by this source. It was widely used and was a large part of the Helghast war machine when defending Helghan, as it powered their Arc Cannons which had stalled the ISA fleet for some time before breaking through. They process the petrusite using refineries, using seawater to cool the reactor and refine the petrusite.
During the events of Killzone 3, large amounts of petrusite became contaminated by the radiation generated after Visari destroyed the capital city of Pyrrhus using captured ISA nuclear bombs. Jorhan Stahl, chairman of the private weapons manufacturer Stahl Arms, gathered this new and more volatile variant of petrusite in order to make even more lethal weapons. The latest weapons and systems that are powered by irradiated petrusite include the StA-5X Arc Cannon, missiles capable of devastating large portions of a planet, and even energy shielding.
Blue Petrusite is highly unstable and if canisters are shot they explode in an electrical explosion and electricity lingers in the area for a while. Green or irradiated petrusite seems to be violently attracted to any form of nearby movement. In Killzone 3 a HAZMAT Trooper is seen gathering the green petrusite and is almost pulled into the crater full of it if not for a Helghast catching him.
Also in Stahl's prison and labs, prisoners are let into a sealed room then a piece of green petrusite is dropped in and the prisoner is destroyed while screaming. Also when the convoy reaches the energy shielding of the crater a soldier begins to panic and the green petrusite kills him, four others, and destroys an Intruder because the soldier moved seemingly drawing the green petrusite beam to him.
Petrusite can come in one color naturally, blue, being the normal electric petrusite. The green petrusite seen in Killzone 3 is irradiated or radioactive, due to the thermonuclear bomb Red Dust being detonated over Pyrhhus, which had vast quantities of petrusite underneath the city. The fact that Helghan had a naturally higher amount of radiation than Human worlds, coupled with a weaponized irratiated petrusite explosion was the catalyst for the Terracide.