The most common reason players purchase diamonds is to build a bigger and better city, and to build it faster. The best 3 items to use Diamonds on in the game for these goals are Premium Expansions, upgrading the Builder’s Hut, and upgrading the Magic Academy.
Premium Expansions: Although Premium Expansions can become quite expensive after purchasing just a few, the first few are relatively cheap and can make a world of difference in growing and advancing our cities.
The extra space means we have more room to add more Residences, Workshops, Production buildings, Culture buildings, and Ancient Wonders which can all help with speeding up our progress and the growth of our cities.
Builder’s Hut: Upgrading the Builder’s Hut will add more builders and allow us to build and upgrade more buildings at the same time thus greatly speeding up our progress.
Magic Academy: Even before the introduction of the Crafting feature, upgrading the Magic Academy was well worth the cost of Diamonds as each upgrade reduces the time it takes us to make the extremely useful Enchantments that can provide the extra coins, supplies, and goods we often need when in a pinch.
Now that Crafting has been added to the game, upgrading the Magic Academy is even more worth the cost of Diamonds, as each upgrade will not only speed up the production of Enchantments, but will also increase the amount of Spell Fragments we get when disenchanting items from our inventory.