Tiberium is a self-replicating, radioactive, flourescent green or ice-blue crystal that can be processed into metals and fuel for both weapons and power plants.
Tiberium is spread by the Scrin as a terraforming agent. Its aggressively self-replicating properties allow it to convert entire worlds into Tiberium planets. It also forcibly replaces the carbon in terrestrial DNA with itself, resulting in mutations.
Tiberium may or may not have a plant-like pod on the bottom, depending on its stage of evolutuon