You won't be able to get an Eviolite in Pokémon HeartGold, because Eviolite is an item introduced in Generation V.
Pokémon HeartGold is featured in the Pokémon Generation IV games; Pokémon HeartGold & SoulSilver (HGSS) are remakes of the original Gold & Silver games released in 1999.
The gameplay follows the same story as Gold/Silver/Crystal. You start in Johto and must travel the area beating the eight gym leaders and then the Elite Four (plus the Champion). After 'completing' the game you are able to visit the original Kanto region and battle its eight gym leaders as well as "Red", a tough trainer found on top of Mt. Silver.
Check out 10 things you might not know you can do in Pokémon HeartGold/SoulSilver at Nintendolife.com.