Board Thread:Dragon City Gameplay Questions & Answers/@comment-4856348-20130827021620/@comment-14284922-20130827030343

The Oil Barrel is due to a current glitch in the game.

Certain crystals, when built, will not be able to be completed on PC once the timer expires. They can however be completed on mobile or with the use of gems prior to the timer running out. This however results in the Oil Barrel being displayed as a collectible item.