A chunk of white phosphorus.
White phosphorus is a material made from a common allotrope of the chemical element phosphorus that is used in smoke, tracer, illumination and incendiary munitions. Other common names include WP, and the slang term "Willie Pete," which is dated from its use in Vietnam, and is still sometimes used in military jargon. As an incendiary weapon, white phosphorus burns fiercely and can set cloth, fuel, ammunition and other combustibles on fire, and cause serious burns or death.
In addition to its offensive capabilities, white phosphorus is also a highly efficient smoke-producing agent, burning quickly and causing an instant bank of smoke. As a result, smoke-producing white phosphorus munitions are very common, particularly as smoke grenades for infantry, loaded in grenade launchers on tanks and other armored vehicles, or as part of the ammunition allotment for artillery or mortars. These create smoke screens to mask movement, position or the origin of fire from the enemy. White phosphorus is used in bombs, artillery, mortars, and short-range missiles which burst into burning flakes of phosphorus upon impact. The aliens' Wedge Ships in Battle: Los Angeles seemed to use this chemical for clearing out pockets of resistance during mop-up operations.