Well, I'm sure don't nobody wanna hear my architect opinions again, but......
One or both of them shoulda known better than to do that installation, Tom. If a competant architect was involved with the designing, he would have known the use of CBU over 3/4 inch plywood over 19.2" joist spacing was not an industry approved procedure even for ceramic tile, let alone natural stone. Couple that with an incorrect CBU installation and I agree with Bill, your chances of even ceramic tile surviving are poor.
If you want your stone (as I presume you were promised by the builder in your contract), you'll need to take up the existing floor at least down to the plywood subfloor, add another layer of plywood, correctly installed, then a substrate of one kind or another, correctly installed, then tile. If that's gonna cause finished floor height problems, we can discuss that, too.
The article Bill posted, by Dave Gobis, is a good one for you to present to the builder and architect. It's simple, clear, and athoritative. Dave has tested a
lot of floors in his time and knows whereof he speaks.
My opinion; worth price charged.