Diamonds can be bought via in-app purchase at the appropriate sub-menu of the Main Menu or directly at the Shop (if you're purchasing Diamonds-priced item without having enough Diamonds).
Diamonds can also be earned as a reward in Time Limited Events, for watching video ads, from Landmark, Daily Rewards, etc.
Diamonds can also be earned as a reward in Time Limited Events, for watching video ads, from Landmark, Daily Rewards, etc.