Latiasite

If held by a Latias, allows it to Mega Evolve into Mega Latias if its Trainer has a Key Stone. Each Trainer can only Mega Evolve one Pokémon per battle.

Unaffected by item negating effects (such as Embargo and Klutz). Item manipulating effects (such as Trick and Fling) cannot give Latias a Latiasite or remove a Latiasite from Latias.

If held by a Latias, allows it to Mega Evolve into Mega Latias when it performs a Spectacular Talent during a Pokémon Contest Spectacular while its Trainer has a Key Stone. Latias will remain a Mega Latias until the Talent Round ends.

In Pokémon Shuffle, the Latiasite allows the player to Mega Evolve Latias.

The player can obtain the Latiasite by passing Level 50 on a Mega Latias Escalation Battle in Weeks 8 and 9 of the event rotation.

The Latiasite was also available as a prize during the Mega Latias Escalation Battles prior to the event rotation system. This Escalation Battle ran on the Nintendo 3DS version from September 28 to October 12, 2015; January 31 to February 14, 2017; and October 24 to November 7, 2017. It ran on the Pokémon Shuffle Mobile from November 24 to December 8, 2015; January 31 to February 14, 2017; and October 24 to November 7, 2017.

The Latiasite enables the following Mega Evolution:

Stone	Pokémon			Type	Effect	Gauge	Max
			Mega Latias	Dragon	Erases Pokémon on a random jagged line horizontally. Actually erases two random jagged lines of Pokémon horizontally. These lines may cross.	22	13

In Omega Alpha Adventure 1, Latios and Latias were revealed to have obtained their respective Mega Stones.