A special stone originally found in the Sinnoh region that can make certain species of Pokémon evolve. It is very tough and has a beautiful sheen.
Special evolution items like the Sinnoh Stone first arrived back with the second generation of Pokémon, and are now required when evolving Pokémon such as Lickilicky and Rhyperior and Tangrowth and Electivire and Magmortar and Togekiss and Ambipom and Yanmega and Honchkrow and Mismagius and Gliscor and Weavile and Mamoswine and Porygon-Z and Gallade and Roserade and Dusknoir and Froslass. When evolving these Pokémon, this item is required in addition to the candy, and is consumed when used.
How to get the Sinnoh Stone
The Sinnoh Stone is obtained from spinning Pokestops and Gyms. There is a very low drop chance of around 1% per spin to receive a Sinnoh 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 Sinnoh 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
The other evolution items that can be found in Pokemon GO are: