Yes, it's true. I posted about this in a thread asking about where to get Tiberium, but I'm glad you started this new one. It deserves its own thread!
I've had several heart cut Tiberium gems transform into the full size spire cut. It's a lot cheaper than cutting it into the small spire and you really get the same end result. The small spire cut it $6,500 and the heart cut is $1,000. Both will transform to a very nice $40,000ish large spire. You don't have to put it in a garden or on the yard for it to happen, though. I just put mine on the floor or on a table. As long as they have some space around them, they'll do it.