I don't think I really did a good enough job of explaining my conclusions. RAW now say's that you can create unnatural stone with a Mu req. I'm not a fan of it and may want not
to use it in my games, because it opens up a can of worms game balance wise.
Either way the rules are the rules. Whether they are house rules or RAW. And whichever rule I choose to include in the setting I can usually come up with a story to explain why Magic works that way.
The writers of the game have done a good job of making magic seem real. Giving us, the players, a sense that we understand how it Hermetic Magic works. Which is impressive because when you look at it there is little explanation as to magic's inner workings. A bit here about platonic ideals, a bit there about natural relationships between things, a bunch of rules about what magic can't do and why magi think they can't so. But of course magi are definatly wrong about some of it and may be wrong about all of it.
Instead most of what "We" know about magic are game rules. This effect does this, this range does that, the numbers add up this way. If you want to do x you need a b c. The narrative as to why it works is largely left up to us. This of course varies from person to person and even saga to saga. This works great right up until new game rules come out. When we find out the stories in our head don't quite match up with the writers ideas.
I try, not always successfully, to remember that how magic works is part of the story. Often I put lot of work into saying something specific about the story in how I house rule things and there effects. (Which my players almost never notice) So I'm pretty used to creating explanations of new rules. So if I can make burning stone permanantly with Cr(Mu)Te(Ig) I can explain it if anyone thinks to ask. One possible explanation for this rule is.
You can create things ex nihlo that don't naturally exist. You do it by adding together parts of the essential nature of different things that do exist together. You need Muto to do this because it works by rewriting things essential nature. Muto magic usually needs an ongoing effect to keep the targets original nature from reasserting itself. But since the target didn't exist before the spell was cast the targets essential nature is set by the spell and sticks.