This Mirror is the one that Lapis Lazuli had been placed into after she ended up caught by Homeworld Gems.
Description[]
An ornate design of silver-blue metallic material comprises the backing and handle of the mirror; Lapis' cracked gemstone was affixed to its back. The mirror itself is a round piece of reflective blue glass with a faint contour of an octagonal gemstone etched on its surface.
The mirror is capable of showing the memories of past events towards those that look into its glass surface, though this seemingly is something Lapis herself will have to willingly do, as she has shown defying Pearl when asked for the Galaxy Warp.
History[]
It is revealed that Lapis was supposed to come to Earth for only a short time but got caught in the middle of the Gem War. She is eventually "poofed" by Bismuth and a Homeworld soldier, mistaking her for a Crystal Gem, imprisoned her in the mirror. Their intention was to hold her captive and gather information about the opposing forces, which Lapis obviously couldn't give, as she was not a rebel gem. When all of the Homeworld Gems fled, she was stepped on, resulting in the damage to her gemstone, and left on Earth by the Galaxy Warp. She was eventually found by Pearl.
It ends up in Amethyst's room sometime after being brought back, and when wondering about the concepts of war, Amethyst brings the mirror out and asks about it. It takes a bit but she does show the Gem War and why the Crystal Gems were involved, repeating these events once Amethyst confronts the others as backup proof. Pearl places the mirror in her gemstone after this.
Pearl gives the mirror to Steven so he can learn about Gem culture and history; it can show its viewer any event it was present for since its creation. Pearl tells Steven they found the item at the Galaxy Warp. As Steven and Lapis begin communicating, she tells him she was purposely imprisoned within the mirror. Steven releases her by removing the gemstone from the back, shattering the mirror.