What do stones do in Pokémon Yellow?

Answer 1

The stones make certain Pokemon evolve. In Generation I, there are five in total:

Fire Stone

Water Stone

Thunderstone

Leaf Stone

Moon Stone

Eevee, Growlithe, and Vulpix evolve using the Fire Stone;

Eevee, Poliwhirl, Shellder, and Staryu evolve using the Water Stone;

Eevee and Pikachu evolve using the Thunderstone;

Exeggcute, Gloom, and Weepinbell evolve using the Leaf Stone;

And Nidorina, Nidorino, Clefairy, and Jigglypuff evolve using the Moon Stone.

Be warned, however; with the exception of Eevee, once a Pokémon uses a stone to evolve, they'll hardly learn any more moves naturally (one or two at most). As for where to find them, the first four can be purchased at the Celadon Department Store (2100 PokéDollars each), and there are five Moon Stones hidden throughout the game: one on Route 2, one in the Rocket Hideout, one in the Pokémon Mansion, and two in Mt. Moon.