The Unity Crystals are featured in My Little Pony: A New Generation, My Little Pony: Make Your Mark and My Little Pony: Tell Your Tale.
History[]
According to Discord, one day, a mysterious wicked unicorn pony (later revealed to be Opaline Arcana in The Isle of Scaly) came along and caused pandemonium by convincing some ponies into hating each other. Soon, all ponykind found themselves divided, which caused a terrible event to happen.
To make sure magic never pitted unicorns, pegasi, and earth ponies against one another, Twilight Sparkle gathered all the world's magic and placed it in three crystals. As long as these crystals were protected, and all ponykind lived in harmony, magic would be kept alive and well in Equestria. In Chapter 4, its revealed the Crystals are conduits which are enchanted with a complex spell made from Unity Magic for protection with the possibility of having Prisbeam Magic inside of them. And the only thing that could possibly drain those crystals is a power of equal magic such as an Alicorn Magic.
Earth pony crystal[]
The earth pony crystal is clear, and shaped like a circle. It's supposed to represent the hooves of an earth pony.
Unicorn crystal[]
The unicorn crystal is purple, and shaped like a horn. It's supposed to represent the horn of a unicorn.
Pegasus crystal[]
The Pegasus crystal is light blue, and shaped like wings. It's supposed to represent the wings of a Pegasus.
Together[]
The three crystals fit together, representing the friendship between all three pony kinds.
