From Desert Operations Wiki
There are 7 different resources in the Game:
Oil, diesel and kerosene is needed to attack other players. This is why the are put in the same category. Oil is a very scarce commodity in the game and it is wise to concentrate on the construction of the Oil Refinery.
The Diamonds can be exchanged for resources, you will also receive a certain amount according to the amount of Buildings you have producing the resources. The amount you receive from the buildings can be seen under the "Income + Expenses".