Hey,
@StrawberryJC good suggestion. I definitely see your point.
However, it has been suggested before. Coins and supplies are the basic resources (cost nothing to generate them). Therefore, there is not an option to purchase a specific amount of tools or coins with diamonds.
However, when you try to level up a building, cater to the tournament, or research a tech, the game will give you the option to pay in diamonds for the resources you are missing, including coins and supplies.
I hope it helps!