One thing is to look at the bottom of the piece. That can be a giveaway as to whether it's jade or synthetic.
I don't know about all, but "most" of the jade used in China now has to be imported. Seems I remember a friend in the jade business telling me that they used up their domestic supply long ago.
As for the 20-ton boulders, it all depends on the quality. The jade has to have the right colors, cannot be fractured, and several other things. Good amounts of quality jade is getting increasingly difficult to find. My friend was telling how they have several hundred tons (maybe thousands - can't remember) of jade, but it's not good enough quality for carving. Therefore, China won't buy it.