The recent Online Competitions for Pokémon Sun and Moon have given thousands of players a chance to take part in action-packed battles, as well as an early opportunity to receive certain Mega Stones for participating. Players in these competitions will continue to be the first to have the opportunity to obtain Mega Stones as participation prizes in Online Competitions. While Mega Stones are time-limited exclusives for participants in Online Competitions, don’t worry if you missed an opportunity to get yours – The Pokémon Company will eventually make these Mega Stones available to all Pokémon Sun and Moon players.
Source: Pokémon Global Link
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