Shiny Lunatone in Pokemon GO is available and will be much easier to catch during the Solstice-themed event. Niantic Labs described the event in their announcement post.
"To celebrate the days getting longer in one hemisphere and the nights following suit in the other, Pokémon GO’s solstice-themed event is returning! Depending on which hemisphere you’re playing in, you’ll be able to encounter and catch different Pokémon during this event."
Shiny Lunatone Pokemon GO: How to Catch
Shiny Lunatone has been catchable for a while, but now it will be easier during the event.
"Lunatone and Solrock will be appearing in the wild around the world during the event. Afterward, Lunatone will be appearing exclusively in the eastern hemisphere, and Solrock will be appearing exclusively in the western hemisphere. Your Buddy will be eager to help you on your adventures! During the event, they’ll give you gift items and Poké Balls more frequently. The walking distance required to earn hearts with your buddy will be halved."
The event will start on June 17 at 10:00 a.m. local time and continue through June 20 at 8:00 p.m. local time.