The crooked half smile is Kell's first response to her distrust in everything the Archmage has said.

"Reasonable reaction, but he actually was telling the truth."

Or what he believes as truth. Kell doesn't hate the Archmage. Between him and the crazy Queen, Ambrose is without doubt a safer bet.

"Magic rock that is believed to be a source of magic here. It is what keeps us tied to this world. Can be quite temperamental and, as you can see around, from time to time its moods shake things up."