Each Arcane Chest drops Arcane Coins and Arcane Stars for a specific Hero.
If Arcane Stars drop out of the Chest for a Hero who already has such a number of Stars, then these duplicate Arcane Stars will be converted into a certain number of Arcane Coins, depending on the type of the opened chest and the number of duplicates received.
There is a scale in the Chest opening screen, which displays the individual progress of opening Chests. With each opened Chest, the scale fills up. As soon as the scale is full, you are guaranteed to receive a good reward, and the scale will begin to fill up again.