Basalz Powder is a material added by Thermal Foundation. It can be obtained by processing Basalz Rods, just like Blaze Powder can be obtained by processing Blaze Rods. However, it can also be created by infusing Pulverized Obsidian with Destabilized Redstone using a Fluid Transposer.
Basalz Powder only has a single use: it is a critical ingredient for creating Petrotheum Dust, and, ultimately, Tectonic Petrotheum.