Obsidian is a material added by vanilla Minecraft. It is the result of Water cooling down Lava source blocks. Thermal Foundation adds a single item made of this material: Pulverized Obsidian. This material is also sometimes dropped when a Basalz is killed.
Obsidian is a somewhat common ingredient in crafting. The material can also be processed in a Pulverizer to produce Pulverized Obsidian. This can be used as an ingredient to produce Hardened Glass or Obsidian Rods. Pulverized Obsidian is also a critical component in the creation of Petrotheum Dust, and, ultimately, Tectonic Petrotheum. Lastly, Obsidian blocks can be molten down in a Magma Crucible to produce Lava.