Originally Posted by psyonic
She can't access her telepathy in diamond form so she needed to be in human form to keep Xavier blocked.. there :P
Actually, she states in the movie that Xavier can't get to her while she's in diamond form. Turning to diamond would have protected her from Xavier, and the fire and debris. There is no other moment in the film where her diamond would have come in more handy, and she didnt use it.
They could have eliminated the diamond form altogether and saved the 4 dollars they spent on making that effect look so awesome.