Attempting to transfigure the soul gem won't work - especially not with the Doctor already distracted. The form of the crystal resists the attempts to change it it, a pulse of energy bouncing back through the magic in rebuke. Blanche senses it happen but ignores it, focusing on Greg instead.

Maybe if Blanche had been less distracted with being furious, he would have more casually been able to keep hold on the gem he's suspending up above. Unfortunately for him, this is not truly the case. Luce's flames sear through the field holding it, just enough that it begins to fall.

But not before the soul flames rake across that portion of Greg's spirit, like a flood of heat that will pour through the lingering connection and into the rest of Greg's consciousness. What's done to that gem is done to him, and it echoes through to his physical reality too. The pain is not unbearable, but it is present.