The Cutie Mark Vault is a storage created by Starlight Glimmer, presented in "Here in Our Town". It held numerous Cutie Marks, including those belonging to the Mane Six, before it was ultimately destroyed by the townsfolk.
The Cutie Mark Vault is a very tall, magically-sealed vault. It is made out of numerous see-through slots, each large enough to fit a Pony's Cutie Mark inside of it. An equal symbol is engraved at the very top of the vault. The entire structure is magically charged by Starlight Glimmer, but is still rather fragile and can easily be broken with enough force.
Judging by the exterior of the design, it can hold up to 112 Cutie Marks at one time.