Photothermic Powder (also called Stralix's Powder) is a type of explosive developed by Stralix.
The powder is made by separately mixing powdered Heatstones and Lightstones with soot, and then mixing these together. It is black-grey in appearance, due to the crystals being covered in soot.
As the ingredients are inexpensive, Photothermic Powder is cheaper to produce than traditional gunpowder, much safer and more effective.
When a Heatstone and a Lightstone are held together, the Heatstone will give off greater heat, due to energy transfer from the Lighstone. If the crystals are powdered, however, then there is a much greater surface area for energy transfer, so more energy is transferred, much more quickly. This causes rapid expansion and ignition of the surrounding air: an explosion.
In Photothermic Powder, the crystal grains are coated in soot, which prevents the energy transfer. However, if the grains are compressed (by an impact, for example), then the soot is ground off to reveal the surfaces of the grains, which then cause an explosion. The same will happen if the grains come into contact with a naked flame.
A Le-Matoran named Aryll seems to have been the first to discover the effect of mixing powdered Heat- and Lightstones, thus destroying his hut. Merror found him and found out about his experiment, the details of which he relayed to his Skakdi friend Stralix.
Stralix proceeded to buy several job lots of the crystals, which he experimented on for a long time before trying to mix them with soot, thus inventing the Powder. He realised immediately the potential of what he had created, and will soon be trying to find a market for it.