I always sent the gems off to get cut right away, so I never realized that they could grow without getting cut! Man, the money I could have saved...then again, I rarely find them in the wild lol.
But for some reason, I could never get more than one to grow at a time. I could set a bunch out around the house, and one of them would grow, but if I didn't put it away in a chest or sell it off, the other ones wouldn't.
But travel does do something to a cut tiberium:
I kind of look on the large spire cut tiberium as my sim's "nest egg." I tend to save them for when I need to buy something really expensive (like a property). Once, I was planning on buying a vacation property, but wasn't sure if I'd have enough money...so I put about 4-5 grown tiberium in my inventory and went on a trip. My plan was that if the property I wanted was less than the value of the tiberium, I'd sell it and buy it. If not, I'd just keep collecting them until I did, so I wouldn't spend the money on something else.
However, after I got to the base camp, all of the tiberium had shrunk to their original size. I immediately returned home to see if they'd go back to being grown, and they didn't. So I set them out around the house to get them to grow again, and none of them ever did.
Another time, I got a tiberium while breaking space rocks in China and I sent it off to get cut when I went back to base camp. I collected it before I left, and when I returned home, that one wouldn't grow either.
This was a long time ago, so whatever caused it might have been fixed (either inadvertently or intentionally), but I won't take the chance anymore lol.