I believe only a Spire Cut Tiberium will grow, and only once per Gem. You leave it out for a few days, with nothing close by (leave about 4 squares in each direction, that should do it), and it will grow, and spike in value from something low like $4000 to obscenely high - $40,000+.
And honestly, I don't know about bigger gems. If you have Tiberium, I think you'd have World Adventures, so Transfiguration would help. I believe there's an article on the guide about it.