"With the usual question, I take it," interrupted Geralt. "Who is the fairest of them all?" I know, Nehalenia's Mirrors are either polite or broken."
In Chapter IV, Alina, a girl from the village of Murky Waters, is said to have owned such a mirror — broken. It is not clear if she broke it herself or if it was always broken. If she did break it, then why she did so, no one knows, but finding the shards and mending the mirror is part of a quest that is required to finish the game.
In the game, the artefact is referred to as a "Mirror of Nehaleni".