A Thunder Stone is a type of Evolution Stone. It can be used (right-click) to evolve certain species of Pokémon, being consumed on use.
It can be obtained as a Ultraball loot special drop, as random loot from Totem Pokémon, as random loot from Structures, and as rare drop from Jolteon, Zapdos and Tapu Koko.
The Shards can also be obtained by mining Thunder Stone ores or by using Forage on stone-based blocks with an Electric-type Pokémon.
It can also be crafted using the following recipe: