Sun Stone - Evolution Item
A peculiar stone that can make certain species of Pokémon evolve. It burns as red as the evening sun.
Special evolution items like the Sun Stone first arrived back with the second generation of Pokémon, and are now required when evolving Pokémon such as Sunflora and Bellossom. When evolving these Pokémon, this item is required in addition to the candy, and is consumed when used.
How to evolve Sunkern into Sunflora
On your Sunkern pokémon's screen, select the evolution through the usual in-game menu. The Sun Stone and candy will be consumed, and you'll have a new Sunflora with hopefully all the right moves.
In Pokémon GO, evolving Sunkern into Sunflora is a lot simpler than some of the other Pokémon games which required your Pokémon to also be traded to another player for the evolution to take place.
How to evolve Gloom into Bellossom
Evolving your Gloom into Bellossom works in the same way as with the other evolution above:
On your Gloom pokémon's screen, select the evolution through the usual in-game menu. The Sun Stone and candy will be consumed, and you'll have a new Bellossom.
How to get the Sun Stone
The Sun Stone is obtained from spinning Pokestops and Gyms. There is a very low drop chance of around 1% per spin to receive a Sun Stone or any of the other evolution item, and there is an equal chance of obtaining each of the 5 currently available items.
There's no way to ensure that the Sun Stone will drop, but with only 4 other items, there's a greater than 50% chance that you'll receive one once you've spun over 250 Pokéstops.
The 7th day streak bonus gives you a very high chance of obtaining one of the evolution item, but it's not believed to be guaranteed.
Other evolution items
As of generation 3, the other evolution items that can be found in the game are: