Chests

Chests
There are six different types of chests in Pocket Mine: Mystery, Wood, Iron, Gold, Diamond, and Rainbow. Each tier requires more keys to open than the previous tier of chests.

Rewards increase in rarity depending on the chest's tier.

Wooden, Iron, Golden and Diamond chests spawn underground.

Mystery chests can only be obtained by inviting friends to the game, or watching an advertisement.

Rainbow chests can only be obtained by crafting it in the factory.

As of the recent update introducing rainbow chests, it is the most difficult to amass mythical (purple) cards. This is because iron chests drop mostly commons, golden chests drop mostly uncommons, diamond chests drop mostly rares and rainbow chests drop mainly legendary cards. There is no chest which mainly drops mythical cards.