There are different types of chests in Elemental Raiders. The rewards received by players depend on the chest quality.
The maximum trophies (PvP Rating) achieved by the account determine how many rewards are given in the chests.
The number of maximum trophies is equal to the lifetime maximum rating of the player. (maximum trophies are structured in the same way as the Arena ratings)
The chest received by a player with a low rating will contain fewer rewards than one with a higher rating.
Chests contain Rune Stones and Skill Copies used to level up that skill.