Diamonds and Gems look very similar in Gems of War. If you are missing gems or diamonds, check the image of what you are looking at. Gems only have one crystal in the image, whereas diamonds have three small gems and are slightly purple.
If you are missing either gems or diamonds you may be mistaking one for the other. Only gems are displayed on the Kingdom Map or home screen, not diamonds.
In order to check your Diamonds, select Games on the Kingdom Map, then Soulforge. Next, select the small Jewels icon near the chat menu in the top left-hand corner of the Soulforge.
Note: You will only have diamonds in your game if the Soul Forge is unlocked by completing Blighted Lands.