Sinnoh Stones are no longer available as Research Breakthrough rewards in Pokémon GO. Niantic has made this change without any clarification, but stay tuned for any updates. However, you can rest assured that these rare items will be accessible by players using different methods. A Sinnoh Stone is described as 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.