Tiberium appears to be a green crystal that emits a faint green mist. It usually appears in large deposits on the surface, usually consisting of one large crystal and lots of small crystals forming around it.
Tiberium is harvested by both sides using a Harvester, which will bring the Tiberium to the Refinery to be processed unto credits, which is then distributed to all players on the same team. It is also poisonous for all infantry with the exception of the Chemical Trooper.
Despite being depicted as mostly organic lifeforms in Tiberium Dawn, Tiberium in Renegade X look like the crystal in Tiberium Sun, even though being set in the first game's time period.