Elemental (Evolution) Stones are the stones used to evolve certain Pokémobs. Some examples are listed below.
All stones can be bought from merchants in Pokecenters, some are dropped by Pokémobs, and most can be crafted.
To use a stone, just select it and right click the pokémob.
Contents
Fire Stone
The Fire stone can be used to evolve Eevee to Flareon.

8 Stones 1 Torch
Thunder Stone
The Thunder Stone can be used to evolve Pikachu to Raichu , and Eevee to Jolteon.

8 Stones 1 Redstone Dust
Water Stone
The Water Stone can be used to evolve Shellder to Cloyster and Eevee to Vaporeon.

8 Stones 1 Water Bucket
Leaf Stone
The Leaf Stone can be used to evolve Eevee to Leafeon.

8 Stones Any Sappling
Shiny Stone
The Shiny Stone can be used to evolve Minccino into Cinccino.

8 Nether Quartz 1 Glowstone dust
Dusk Stone
The Dusk Stone can be used to evolve Misdreavus into Mismagius

8 Soul Sand 1 Torch
Dawn Stone
The Dawn Stone can be used to evolve a male Kirlia into Gallade.

8 Blue Dye 1 Torch
Moon Stone
The Moon Stone can be used to evolve Clefairy into Clefable.

Lunatone found in Caves and Meteor Areas except in day time
Sun Stone
The Sun Stone can be used to evolve Sunkern into Sunflora.

Solrock found in Caves and Meteor Areas except at night
Packed Ice (Ice Stone)
Currently there are no Ice Stones in the game so Pokémobs that use Ice Stones to evolve use Packed Ice instead. Alolan Vulpix uses it to evolve into Alolan Ninetales, and Alolan Sandshrew to Alolan Sandslash.

9 Ice
Ever Stone
Not an evolution stone. Prevents evolution if held.

8 Stone 1 Obsidian