A rock that can make certain species of Pokémon evolve. It looks like a crown.
Special evolution items like the King's Rock first arrived back with the second generation of Pokémon, and are now required when evolving Pokémon such as Politoed and Politoed and Slowking and Slowking. When evolving these Pokémon, this item is required in addition to the candy, and is consumed when used.
How to get the King's Rock
The King's Rock is obtained from spinning Pokestops and Gyms. There is a very low drop chance of around 1% per spin to receive a King's Rock 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 King's Rock 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
The other evolution items that can be found in Pokemon GO are: