Mortal Mirror

The Mortal Mirror was a mirror-like object that was created by the Darksmiths of Karagula. (PROSE: The Colour of Darkness) It reflected a darker but almost identical realm (or dimension) and could trap people in that dimension. If a person trapped in the mirror saw their reflection, their DNA was replaced with one made of a translucent matrix based on silicon, essentially making a person's body living glass. (PROSE: Martha in the Mirror)

Governor Pennard, the governor of Castle Extremis, gave the mirror to Grieg as a reward for strategising the recapture of Castle Extremis. However, it was actually a trap set by Pennard, who believed that Grieg was the only one who could overthrow the Third Occupation. (PROSE: Martha in the Mirror)

The Doctor noticed it was not a real mirror because he would have seen the little girl with the red balloon, who was trapped in every mirror. (TV: The Family of Blood, PROSE: Martha in the Mirror)