How do you get the diamond key to the door in Virtual Villagers, The Lost City?

The diamond is located in the fire. After the fire has been going for over 10 hours (I had to play on normal speed for a day), you will eventually see sparkles around it. When the fire goes out the diamond will appear in the fire pit. The diamond will be too hot to pick up so you will have to drag your villagers to the waterfall about 10 times and they will pour water on the diamond to cool it down.