Here's a brief sample from GameReplays.org's latest article, The Mystery of Tiberium:
For all the commanders playing the Command and Conquer series for over 10 years, harvesting Tiberium has always been a major economic factor of the game, allowing players to build their army in order to finish off their opponent. Has anybody actually wondered what these green glowing crystals comprise of? How come they grow time and time again for harvesting? How can they be used as resource for building or be used to create weapons of mass destruction? Why is exposure to Tiberium harmful and why are there humans who are immune to Tiberium exposure? Both Westwood and EA did not reveal much about Tiberium after the release of Command and Conquer 4, even though it is the root of the Command and Conquer's Tiberium Series. This is a fan-based article on Tiberium which should help you analyse the information written in the C&C wiki, and showcase N00bKefka's own form of theory in conjunction with Dr. Mobieus' and Kane's theory.
History of Tiberium
The name "Tiberium" was derived from the Tiber river in Italy, the place where Tiberium was first discovered back in 1995. Kane claims that he discovered Tiberium first and named it under the Roman Emperor, Tiberius Julius Caesar Augustus. The Scrin on the other hand call it Ichor. In Greek Mythology, it is the blood of the Gods, and is also poisonous to mortals. The Scrin use the name "Ichor" because they feel that they are superior to humans. Secondly, Tiberium is vital to their life but yet they is harmful to humans. We all know that Tiberium came crashing to Earth as meteor, but nobody knows which part of the universe the Tiberium came from. One thing for sure, it did not come from the Scrin.