Evolution Stones (commonly known as Elemental Stones) are crystals with the power of a certain element that instantly evolve certain Pokémon they are used on. There are currently nine different stones for evolving Pokémon. Fire, Water, and Thunder Stones evolve Pokémon of the corresponding Pokémon types, for example Vulpix, Staryu, and Pikachu, respectively. The Moon Stone evolves fairy-like Pokémon, like Clefairy and Nidorina. Leaf Stones evolve Grass-types, such as Exeggcute and Weepinbell. The Sun Stone, introduced in Pokémon Gold and Silver, evolves the plant Pokémon Sunkern and Gloom. Pokémon Diamond and Pearl added Shiny, Dusk, and Dawn Stones: Shiny Stones are used to evolve Togetic and Roselia, Dusk Stones are used to evolve Murkrow and Misdreavus, and Dawn Stones are used to evolve female Snorunt and male Kirlia.
There are a handful of other stones related to evolution. The Everstone prevents the Pokémon that holds it from evolving. The Sun Shard and Moon Shard (found in Pokémon XD only) simulate the effects of Day and Night, which allows a happy Eevee to evolve into Espeon or Umbreon. When a Slowpoke or Poliwhirl holds the King's Rock and is traded, it will evolve into Slowking or Politoed, respectively. When a Happiny holds the Oval Stone and levels up, it will evolve into Chansey.
Stones
Water Stone
The Water Stone is a special elemental stone, introduced in the 1st Generation Pokèmon games, which when used on certain Pokèmon makes them evolve. It is an elipitcal-shaped stone that is blue in appearance, with bubble insignias inside of it, and usually makes Water-type Pokèmon evolve.
Thunder Stone
The Thunder stone is a special elemental stone, introduced in the 1st Generation Pokèmon games, which when used on certain Pokèmon makes them evolve. It is an elipitcal-shaped stone that is yellow in appearance, with a thunderbolt insignia inside of it, and usually makes Electric-type Pokèmon evolve.
Fire Stone
The Fire Stone is a special elemental stone, introduced in the 1st Generation Pokèmon games, which when used on certain Pokèmon makes them evolve. It is an elipitcal-shaped stone that is orange in appearance, with a fire insignia inside of it, and usually makes Fire-type Pokèmon evolve. This stone is like a shape of a diamond but it is color red.
Leaf Stone
The Leaf stone is a special elemental stone, introduced in the 1st Generation Pokèmon games, which when used on certain Pokèmon makes them evolve. It is an elipitcal-shaped stone that is grey in appearance, with a leaf insignia inside of it, as if it were a fossil, and usually makes Grass-type Pokèmon evolve.
Moon Stone
The Moon stone is a special elemental stone, introduced in the 1st Generation Pokèmon games, which when used on certain Pokèmon makes them evolve. It is an elipitcal-shaped stone that is grey in appearance, looks somewhat like a moon rock, and usually makes Fairy-Group Pokèmon evolve.
Sun Stone
evolves togetic into togekiss and roselia into roserade
Dawn Stone
The Dawn Stone is peculiar stone, introduced in the 4th Generation Pokèmon games, that will only evolve Pokèmon of a determinate gender (i.e. a male Kirlia can evolve into Gallade and a female Snorunt can be evolved into Froslass). It is a round stone, sky blue in appearance, and actually looks similar to an opal.
Dusk Stone
A dusk stone is a very odd stone which was introduced in the 4th generation games. It evolves Misdreavus into Mismagius and Murkrow into Honchkrow.
Shiny Stone
The Shiny stone is a special elemental stone, introduced in the 4th Generation Pokèmon games, which when used on only certain Fairy-Group Pokèmon (i.e. Togetic into Togekiss, or Roselia into Roserade) makes them evolve. It is an elipitcal-shaped stone that is transparent in appearance, with a ball of light inside of it.