Also change the information, becaues if you put "fuel" then it will say like this:
Fuel
Gold is mined from gold mines.
To change the information go to Advanced / Gameplay Constants / Text - General - 'Gold is mined from gold mines' change that for the thing you want.
The icon is in Advanced / Gameplay Constants / Icon - Gold.
Just change that, and if you want to change lumber or food you can also change there.