A person's mind is something that can be quite fragile, particularly if the person happens to be a cutie in a Crapsack World. The iconic fragile thing in fiction seems to be glass - very shiny and very fragile. This leads to it and its shattering being used as a metaphor for an individual's broken sanity.
A popular variant of this is the House of Broken Mirrors.
- Occurs during Asuka's Mind Rape in Neon Genesis Evangelion at the end of a repeated montage.
- Played with in Yu-Gi-Oh!, where the Other Yugi shatters Kaiba's mind after defeating him at the end of the Death-T arc. In this case, it's intended to give Kaiba a chance to regain his sanity as he puts the pieces of his mind back together.
- It's very similarly played with in Berserk, during the ritual that Schierke and Farnese perform in order to restore Casca to normal. Within her dream world, they discover that what's left of Casca's sanity is represented by a broken doll. And as they progress through her memories, the doll is gradually pieced back together.
- A Diplomatic Visit: In chapter 11 of the third story, Diplomacy Through Schooling, Tirek tries to grasp the true might and magic of the Lady of Magic. He not only fails, it leaves his mind broken.
- The epilogue of the fourth story, The Diplomat's Life, reveals that Adagio's overuse of her Mantle to sustain herself in the human world had a nasty side-effect when she lost it, as her mind eventually broke. She was returned to Equestria and checked into a charity mental hospital as a result.
- One of the most frequent images in Sucker Punch is a glass breaking, once just before Babydoll's sister dies (from a light source) and the second time when Blondie dies.
- In May, the Sanity Slippage of the title character is represented by the slow cracking of the glass case she stores her cherished doll Suzie in. It finally breaks when she takes Suzie to a school for blind children that she volunteers at, as they fumble in their attempts to get Suzie out of her case and cut themselves on the broken glass. It's all downhill from there.
- In Doctrine of Labyrinths, Felix's mind is shattered alongside the Virtu in the first book during his rape.
- One very well-known literary usage is in The Glass Menagerie, where the glass unicorn's horn breaking off symbolizes Laura's breakdown upon discovering Jim's engagement.
- In Psychonauts 2, Ford Cruller's fractured psyche is represented as a broken mirror, and putting him back together requires finding one of the shards of the mirror in each of his mental worlds.
- In Narbonic, one of the lenses in Dave's glasses cracks as he descends into madness. Later he gets a new, uncracked pair, but the madness is still there, just under Dave's control.