One FMV in Command & Conquer reveals the known constitution of Tiberium as:
In other words, Tiberium consists mainly of hydrocarbons. Just for you, I did a little research on these compounds, the chemical formulas, and therefore relative formula masses (in g/mol), of which are as follows:
Methane CH4 16.0416
Sulphur S 32.06
Naphthalene C10H8 128.1632
Argon Ar 39.95
Isobutane C4H10 58.119
Xylene C6H6(CH3)2 108.1748
Multiplying each by their relative percentages and adding together, we see that one mole of Tiberium weighs, on average, 134.645772g plus the unknown 29%. That's quite a complex molecule, as is fitting for something like Tiberium which, despite its extra-terrestrial origins, is supposed to be somewhat plant-like. Indeed, Westwood did their research on this because the prevelence of carbon and hydrogen is just what one would expect in a biological entity.
The FMV also provides another set of data, although it's unclear what these are the compounds for. Let me know if you have any inspiration on this:
As mentioned above, Tiberian Sun introduced blue Tiberium, but no such data is available for this. It does seem to be worth more, and is explosive, so I would be interested to hear any estimates for its content.