I know that this has been posted to sci.materials before, but this sure is interesting!
Carnegies's process sounds a little like the sintered diamond approach described in this document
These two processes remind me of glacial ice forming from compacting snow. In the CVD case, are the tiny voids between crystals geting mashed out? How does this result in perfect cleavage? Sintered diamond doesn't have perfect cleavage to be sure, but I wonder if a sintering process applied with 300kBar and 4000C could make gem crystals..