Diamonds
Diamonds are a late-game resource since you can only legitimately own them by constructing the Diamond Depot which requires the Command Center to be level 6. You can gain Diamonds though but they won't be stored. Diamonds are used to quickly level up Heroes through the "Quick Level Up" function upon reaching Command Center level 6 and building a Diamond Depot, training a select number of units, etc..
Diamonds can be obtained from rewards are placed in the Inventory so it is a good idea to keep them there since players target diamond reserves as well. They can be obtained from PvP battles, League rewards, landing zone rewards, the Diamond Miner which unlocks at Command Center level 7, etc..