Diamonds are the third currency inside the world of LoMM. They are used to speed up things that are purchased with Gold and Crystals. Whenever you don't have enough for a purchase or want to speed things up, Diamonds are the way to go.
The may source of Diamonds in game is through purchasing in the recharge section as you can see in the picture, but these amazing gems can also be obtained through events and quests following the example below.
Just like Gold and Crystals, your Diamonds can be checked on the top right corner of the screen, but there's no buildings needed to store or fabricate them.