The Z-Ring is an all new item introduced in Generation VII. Alongside a Z-Crystal, it can trigger a Z-Move.
|| A mysterious ring that enables Pokémon to use Z-Power. It requires both the willpower and the physical power of the Trainer wearing it.
The Z-Ring is technically a white Mega Ring but instead of holding a Key Stone, it holds a Z-Crystal.
In the game the Z-Ring is first seen in it's raw form as a Sparkling Stone. Then, given to the player character by Hala when he's done refining it after the player character has defeated Hau.
The basic setup of a Z-Ring
A Toy Z-Ring and Pikachu from EB Games Australia