If held by a Latios, allows it to Mega Evolve into Mega Latios 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 Latios a Latiosite or remove a Latiosite from Latios.
Outside of battle
If held by a Latios, allows it to Mega Evolve into Mega Latios when it performs a Spectacular Talent during a Pokémon Contest Spectacular while its Trainer has a Key Stone. Latios will remain a Mega Latios until the Talent Round ends.
The player can obtain the Latiosite by passing Level 50 on a Mega Latias escalation battle in Weeks 4 and 5 of the event rotation.
The Latiosite was also available as a prize during the Mega Latios escalation battles prior to the event rotation system. This escalation battle ran on the Nintendo 3DS version from November 24 to December 8, 2015; April 26 to May 10, 2016; June 27 to July 11, 2017; and January 9 to 23, 2018. It ran on the Pokémon Shuffle Mobile from April 26 to May 10, 2016; June 27 to July 11, 2017; and January 9 to 23, 2018.
The Latiosite enables the following Mega Evolution:
Data for Latiosite exists within the code of Pokémon X and Y, despite the Mega Stone not being officially introduced until Omega Ruby and Alpha Sapphire, allowing it to be transferred to and used in X and Y. It shares this trait with the Latiasite.