You can get diamonds by:

• Completing achievements
• Purchasing them from the Gift Shop
• Extracting them from rifts, using your warper.
• Watching in-game ads in diamond rifts. Please bear in mind that you can only watch a limited number of these every day, and you can only watch them while you are using a diamond rift.
• Opening loot boxes.
• Completing sets in the Tailor of Fortune. 
• Selling items in the recycler. 
• Using the free diamond offers in the Gift Shop.