The Stone Coffer are dragon-like stone statues found in Tanglevine Jungle. There are two located next to the ruins (one with blue eyes and one with green eyes). When clicked on they provide a host of games such as match the jewels and Flash Cards. Players are rewarded with coins once these games are completed.
The reward for one of the Daily Events was a Stone Coffer that you could place in your home.
The stone coffer with green eyes contains a secret staircase to the Onakasi Treasure Room. You need to move your mouse cursor repeatedly over the coffer to make the staircase appear. If you get friends to help you it goes faster.