My otherwise useless sub to PFK magazine has actually been a little useful, there was an article in there about marine aquarium decor. In it there is a description of "imitation live rock", which is apparently made of "live rock like materials" and then coated in a reef safe coating in either pink or purple. Apparently some even comes pre-seeded with bacteria. Supposedly it's the rock of choice these days, as it's easier to work with than real live rock, as it comes in specific shapes.
Still, if it was sold as live rock, and not imitation live rock, that's a heck of a dodgy move by the seller.