You can get diamonds from the Diamond Box multiple times per day. You will also receive a number of diamonds every time you level up, as well as when you complete other tasks like solving investigations.

You can use diamonds to:

• Buy boosters to help you with hidden object scenes and match-3 games
• Complete a challenging scene if you need extra time or moves
• Get missing spectral binders
• Purchase sale items in Blitz Offers

To see how many diamonds you currently have, check the top of the main map screen in the middle.