Key Stone is an item in the “Gameplay” category (Key Items).
In-game descriptions
| Sun / Moon · Ultra Sun / Ultra Moon | A stone filled with an unexplained power. It makes Pokémon that battle with a Mega Stone Mega Evolve. |
|---|---|
| Let’s Go, Pikachu! / Let’s Go, Eevee! | A stone filled with an unexplained power. It reacts to Mega Stones and makes Pokémon Mega Evolve. |
| Sword / Shield | A stone filled with an unexplained power. It makes Pokémon that battle with a Mega Stone Mega Evolve. |
Where to find it
Not placed on the atlas maps (so far: Red/Blue and Generation 3).
Sources : PokeAPI (items, item_flavor_text, machines) · atlas · pokeemerald species_info.h