In the game, you may get chests that contain collection items. As a rule, these chests and their contents are linked to a specific game event.
To see the list of items and their drop rate, you can press on the chest image and then on the blue "i" information button.
Three kinds of chests should be particularly noted: Marble Chests, Carved Chests, and Rare Chests. They contain items from all collections except for event ones.
The drop rate of each collection item from Marble Chests is 0,0668%
The drop rate of each collection item from Carved Chests is 0,1335%
The drop rate of each collection item from Rare Chests is 0,2002%