It was recently announced on the official Pokémon website that there will soon be an event in Pokémon GO that will be centered around Rock-type Pokémon. The event will start this Thursday, May 18, and will continue through May 25.
Among other things, Rock-type Pokémon will have increased spawn rates during the week of this event. If you have a Metal Coat, this event will allow you to catch enough Onixs to use the Metal Coat to evolve an Onix into a Steelix. You will also have a higher chance of encountering the pseudo-legendary Tyranitar or its pre-evolutions. Other Rock-type Pokémon that may have increased spawn rates include Geodude, Graveler, Golem, Rhyhorn, Rhydon, Omanyte, Omastar, Kabuto, Kabutops, Aerodactyl, Sudowoodo, Shuckle, Magcargo and Corsola, although Corsola will only appear in places between 26° and 31° latitude.
In addition to the Rock-type spawn rate increases, there are several other parts of this event. Your Buddy Pokémon will find a Candy at four times the rate it usually would. This means that if your Buddy Pokémon usually takes 1km to find a Candy, it will find one in .25km, a 3km Buddy Pokémon will find a Candy in .75km and a 5km Buddy Pokémon will find one in 1.25km. Additionally, PokéStops will give you more items than usual and Poké Balls will be available from the game’s in-game store for 50% their usual price. Lastly, during the event a new piece of clothing will be purchasable called the Explorer’s Hat, which can be purchased with PokéCoins, the game’s in-game currency.